fix binding to vaos
add missing include
don't replace shaders on duplicate setup() call

fixes #408
refactor vao object to use acutal OpenGL IDs rather than object pointers
base static vertexbuffer on buffer object
Merge pull request #415 from mapbox/fix-vao

Line drawing slowdowns on OS X
coalesce texture uploads at the beginning of a frame
allow crosstile labels and add `symbol-avoid-edges`
use pimpl for collision in tile_parser
Merge pull request #423 from mapbox/coalesce-texture-uploads

Upload textures at the beginning of a frame
Merge pull request #419 from mapbox/crosstile

allow crosstile labels and add `symbol-avoid-edges`
remove support for composite layers
remove trailing whitespace + use local image sprite
fix stylesheet indentation
remove composite enums from tests
Merge pull request #420 from mapbox/remove-composite

remove support for composite layers
pin to mapnik-packaging@848c6e8d6692a9 for now to protect against upstream refactoring
install node in cross-platform way + fix mapnik-packaging checkout
update mapnik-packaging to work around openssl problem

 - refs https://github.com/mapnik/mapnik-packaging/commit/6d7e83e8042a306d847a1e0b82b9bbe8eb76ccdd
update README with more iOS info