added support for new HiDPI retina Macs (closes #303)

Via the new property "supportHighResolutions" on the Starling instance.
added check if 'stage.contentsScaleFactor' is available
Fixed bug where vertexID was being used instead of zero

getBounds() function would not work when specifying numVertices that is smaller or equal to vertexID. This was because vertexId was being used incorrectly in the for loops. Zero should have been used instead
Merge pull request #309 from hamidhomatash/patch-1

Fixed bug where vertexID was being used instead of zero
moved up 'formatText' so that native text field will be drawn correctly (closes #313)
added 'AssetManager.enqueueWithName'
added 'checkPolicyFile' property to 'AssetManager' class (closes #314)

To allow loading assets from remote domains.
small API documentation fixes
setting pivotX/Y to zero in transformation matrix setter (closes #315)
added test for transformation matrix setter with pivot
Merge remote-tracking branch 'upstream/master'
added progress accessor to Tween

find a Tween's progress, as determined by its transition function, from
0 to 1.  previously, this was calculated only within animate() (and was
inefficiently calculated separately for each tweened property).