This allows a page to have multiple placeholders, since we're no longer
tied to unique IDs.

To improve organization with the increasing number of examples, every
example page is now the index of its own subdirectory, which contains
all the files specific to that example.

Linked the errorbars example to the examples index page, and cleaned up
both the example and documentation at the top of the plugin.

As in http://code.google.com/p/flot/issues/detail?id=215