use availability macros for better compatibility
Break strong reference cycle between map and it's delegate.
Add missing dispatch_release calls
Break strong reference cycle in RMQuadTreeNode.
Don't call dispatch_release if dispatch_queue_t  is declared as Objective-C object.
Merge pull request #503 from ilmarion/issue221

Fix several memory issues.
Added max database cache file size configuration option
Added db cache max file size configuration option
Merge branch 'disk-cache-size' of github.com:Hitta/mapbox-ios-sdk into develop