Allow VectorSource to load and parse data

This adds url, data and parser options to the source, and makes
EPSG:4326 the default projection. It also adds a prepareFeatures
method, which is used to load/parse data once the target
projection is known.
Removing unused methods and changing getFeaturesObjectForExtent

With this change, getFeaturesObjectForExtent may return null if
the source does not have data loaded for the provided extent.
A callback can be passed to getFeaturesObjectForExtent to get
notified when the requested data is available.
Updating examples to use the new API
Addressing @elemoine's review comments
Merge pull request #870 from ahocevar/vector-api

Give VectorSource the ability to load and parse data