Image service identify

Resolves #296

Had a few [stumbling blocks](https://github.com/tomwayson/esri-leaflet/commit/6f05988365da184090424ff4a35faf0f48885a18), so I didn't get in as much as I'd like (namely `ImageMapLayer.bindPopup()`).

That said, this does what most people want to do w/ an image service identify, click a point on the map and get the pixel value at that location.

The new example page is works well and also added an API reference page.

![screen shot 2014-08-07 at 3 35 06 pm](https://cloud.githubusercontent.com/assets/662944/3850275/32394496-1e83-11e4-9790-6511f0eeb829.png)


Lots of changed files due to renaming `L.esri.Tasks.Identify` to `L.esri.Tasks.IdentifyFeatures`, but all tests are passing so should not be any regressions.

 I'm happy w/ this for beta6. After merge will add post-beta6 issues for `bindPopup()` and `returnCatalogItems` as future enhancements.