Trying to track down a loading problem.
Added a tile unload callback for tile sources.  Fixed an ordering problem with tile sources that returned immediately.
xcode6 compilation fixes
Fixed importance scale in single (or multi) level loading cases.
Merge pull request #222 from trailbehind/feature/xcode6

Xcode6 compilation fixes
Was accidentally overriding low performance logic.
Exposing border pixel size.  Moved .bat to .sh because... duh.