the TextField inside the Button class now has 'batchable' enabled (closes #404)

Buttons typically only have very short texts, so this should be a useful default.
added 'Starling.registerProgramFromSource()' method
now caching shader programs of FragmentFilters (closes #410)
optimized 'set numVertices' loop in VertexData (closes #406)
Merge branch 'master' of https://github.com/PrimaryFeather/Starling-Framework