Fix bug at modify interaction construction time

This fixes a bug that occurs when the modify interaction is configured with a non-empty collection of features.
Export ol.source.TileImage
Add ol.coordinate.sub
Add ol.View2D#rotate
Merge pull request #1972 from elemoine/modifyinteraction

Fix bug at modify interaction construction time
Export ol.View2D#rotate
ol.animate.rotate takes an optional anchor
Add rotateAroundRome to animation example
Use ol.View2D#rotate in Interaction
undefined may be passed to ol.animation.rotate
Merge pull request #1942 from elemoine/export-tileimage

Export ol.source.TileImage
Merge pull request #1975 from fredj/rotation-anchor

Rotate around arbitrary coordinate
Automatically skipFeatures on select’s collection changes
Required return from event handler
Replace typedefs with externs

Since the compiler leaves externs untouched and learns the used
properties from object literals, this replaces the otherwise
required typedefs just fine.
Merge pull request #1974 from tonio/selectfeature

Automatically skip features on select’s collection changes
Merge pull request #1976 from tschaub/pointerup-return

Required return from event handler.
Merge pull request #1971 from ahocevar/externs-instead-of-typedefs

"Cannot read property 'AttributionOptions' of undefined" error
Add a comment to build.py
Merge pull request #1977 from elemoine/comment

Add an olx.js related comment to build.py