Vector source methods

For getting features, we currently have the following:

 * `getAllFeatures`
 * `getAllFeaturesAtCoordinate`
 * `getAllFeaturesInExtent`

To me, the `All` part is a bit redundant.  What do people think of the following?

 * `getFeatures`
 * `getFeaturesAtCoordinate`
 * `getFeaturesInExtent`

Alternatively, we could leave the `getAll*` methods (perhaps for internal use) and expose the following API method:

 * `getFeatures(opt_coordOrExtent)`

I know this makes for looser type checking, but we wouldn't have to use it internally.

Really, I think a method like `getFeatures` should take an optional filter.  But I don't think we're headed there.