Added hidden boolean to Tanks
Tank hides in trees

* Added hidden parameter to Tank
* Tanks check to see if surrounded by trees on all sides at each update,
and are hidden if so.
* Pillbox controller changed to only fire at non-hidden tanks.
Fixed bugs in integration tests
Make TileUtil.isValidTile public

* Added unit test, included in Tank tree check.
Fixed formatting in TileUtil.java
Tank now checks current Terrain for trees
Added javadoc comment to TileUtil.isValidTile
Merge pull request #306 from BU-CS673/Aaron_HiddenTanks

Tanks now hide in trees
Fixed merge conflicts