Factor out ol.source.Vector#addFeatureInternal
Add ol.source.Vector#addFeaturesInternal
Add ol.source.Vector#addFeatures
Use addFeaturesInternal in ol.source.Vector constructor
Change ol.source.Vector#removeFeatureInternal from private to protected
Clear null geometry features in ol.source.Vector#clear
Use addFeatures in ol.source.VectorFile
Merge pull request #1414 from twpayne/vector-api-add-features

[vector-api] Add ol.source.Vector#addFeatures