Legend improvements

Making the Legend button slide out a panel with an accordion section for each data source.  If the data source's legend is an image, it is displayed inline.  If it's an HTML page, a link allows the user to open the page in a separate tab.  If the data source doesn't have a legend, the user sees "No legend available for this data source."

![image](https://cloud.githubusercontent.com/assets/924374/4280737/9a15e9c4-3d33-11e4-84d1-2350c967d489.png)

Fixes #124.