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/))

API.md

+**Table of Contents**
+
+- [Flot Reference](#flot-reference)
+- [Data Format](#data-format)
+- [Plot Options](#plot-options)
+- [Customizing the legend](#customizing-the-legend)
+- [Customizing the axes](#customizing-the-axes)
+- [Multiple axes](#multiple-axes)
+- [Time series data](#time-series-data)
+- [Customizing the data series](#customizing-the-data-series)
+- [Customizing the grid](#customizing-the-grid)
+- [Specifying gradients](#specifying-gradients)
+- [Plot Methods](#plot-methods)
+- [Hooks](#hooks)
+- [Plugins](#plugins)
+- [Version number](#version-number)
+
 # Flot Reference #
 
 Consider a call to the plot function: