Handle skipped features when setting the map

When a Select interaction is removed from the map, it should
remove its selected features from the map's skippedFeatures
collection. When it is added to a map, it should add them.
Rename ol.source.VectorFile to ol.source.StaticVector
Factor out ol.source.FormatVector
Rename ol.source.FormatVector#loadFeatures to loadFeaturesFromURL
Load features before rendering
Add ol.loading
Add ol.source.RemoteVector
Load features on demand in vector-osm example
Export ol.format.OSMXML
Add projection argument when loading features
Rename ol.source.RemoteVector to ServerVector
Rename ol.loading to ol.loadingstrategy
Replace extentUrlFunction with generic loadingFunction
Use a BBOX loading strategy by default
Rename loadingFunction to loader
Rename loadingStrategy to strategy
Make tile URL functions more general
Separate out feature reading and feature adding
Add ol.source.TileVector
Add tile-vector example