Add table of contents

The reference document is quite large; a table of contents would help readers navigate and find topics. (TOC was generated via [DocToc](http://doctoc.herokuapp.com/))

Updated threshold plugin to work with 'above' operator. Resolves #1059.

Lots of people ask about implementing tooltips; hopefully enabling it by
default will help them find it more easily.

These changes were used to test the axis innermost property fix, and
should not have been committed.

An axis is innermost when it's first among those not just in its
direction, but also on its side, i.e. left/right, of the plot. So the
inArray check should be against the samePosition list, not all.
Resolves #1056.

It doesn't make sense to calculate the innermost property only when no
tickLength has been provided; the two have little to do with each other.

Resolves #1032.  Previously it was impossible to draw the same text,
with the same style, in two different locations, because the second
would end up using the first's cache entry, which only ended up moving
the element to a new position.

Now each cache entry holds a list of positions at which the text
appears, creating clones of the original element for each position
beyond the first.

Resolves #1030.  The performance impact should be low, and is the
correct thing to do in just about all cases where a plot is being
redrawn.

Resolves #1033.

 Removed executable permissions from jquery.flot.selection.js

Flot 0.7 calculated x-axis label dimensions by assigning each label a
fixed width, then measuring the height as determined by the browser.  A
side-effect of this technique is that x-axis label divs received a fixed
width.  The rewrite of the text system in 0.8 accidentally removed this
feature; this patch restores it.

The optional width parameter establishes a maximum width for the text's
bounding box, forcing it to wrap when it reaches that size.

Regression on pie plugin in the data format allowed

Patch for colors array, issue #1031

Per request from dnschnur (https://github.com/flot/flot/pull/1034#issuecomment-17119203), I have: 
- Added a comment explaining the purpose of the new check/override
- Changed the structure to match preferred standard structure

Add links between API.md and README.md

Remove $.isNumeric() for compat with jQuery < 1.7. Fixes #1026

(Realted to flot issue #1031: https://github.com/flot/flot/issues/1031)

Currently, if the user declares a custom color palette with less than 5 colors (say, n), $.extend(true, options, opts) only modifies the first n colors of the default palette and leaves the last 5-n in place. When the number of series is >n, colors are used that are not part of user-defined palette, contrary to description of colors array in API.

This line overrides the extended colors array and replaces it with exactly the user-defined colors array, when present. Afterwards, the user color palette is respected by the auto tinting/shading system for when there are more series than colors.

Remove some incorrect else-if logic in time plugin.

Avoid propagating mouse wheel events when they're first caught by the navigate plugin.