Only set interacting hint when modifying the view
Merge pull request #1277 from tschaub/interation-hint

The ol.interaction.Drag subclasses set interacting hint as needed.
Protect from null vertexFeature

When dragging the map before a vertexFeature was created, it can
be null, so we cannot get its renderIntent.
Merge pull request #1284 from ahocevar/null-vertexfeature

Protect from null vertexFeature
Vector layer/source refactor

This moves the feature cache from ol.layer.Vector to ol.source.Vector.  These are the minimum changes required to maintain the existing functionality and make tests pass.  More refactoring to come.
Private source methods
Remove unused method
Moving all style functionality from source to style
Move render intent enum to feature
Separate load requests from feature requests

This separates the action of requesting an extent to be loaded from the action of requesting cached features.  The renderer (or any other consumer of a vector source) calls load to request a data extent.  A `featureload` event fires when new features are loaded.  The renderer (or any other consumer) separately asks for cached features given an extent.  This vector source only loads features once, but this separation will also work with sources that make multiple requests for data in different extents.

This also removes the `data` option from the vector source in favor of a `features` option.  Since we no longer have shared data structures for geometries, people can manually create features and pass them to a vector source.  The `addFeatures` method is exported as well.  This is used to add features to a source that don't have a representation on the "remote" (or server).