Allow tiles to be skipped in draw loop when they are null (i.e. loading)
Provide getters for the number of vertices and indices in a VboMesh
Remove cached tesselator instance and instead instantiate tesselators on demand (tesselator is not thread-safe)
Modify PolygonStyle methods for thread safety; No member values can be written within addData
Modify DataSource to retrieve only one tile at a time, with the intention of spawning this process in background threads
Use c++11 async functions to fetch and build map tiles asynchronously from TileManager;

TileManager spawns a new task for each tile request using a lambda, then polls the status of incoming tile tasks during each update loop
Delete tesselator instance after usage (was being leaked)
DataSource now uses cached tile data if present;

Currently, tile data is all effectively indefinitely cached
Cleaner iterator usage when removing elements from a collection while iterating
Merge branch 'master' into async-tiles;

Reorganized some of the new and renamed methods into the newly separated DataSource subclasses

Conflicts:
	core/src/dataSource/dataSource.cpp
	core/src/dataSource/dataSource.h
	core/src/tileManager/tileManager.cpp
Discard Json tile data immediately after tile construction (i.e. zero caching)
Merge pull request #16 from tangrams/async-tiles

Async tiles