"skip install" for lib Xcode targets for easier app archiving
fix png_reader
image-readers: only register readers that are used

create_png_reader(char const* data, size_t size)
create_jpeg_reader(char const* data, size_t size)
add note about access token & where to find Linux binary
Merge branch 'master' of github.com:mapbox/mapbox-gl-native
simplify image_readers creation implementation

* remove static callbacks registration
* link png_reader/jpeg_reader into mbgl-linux lib
ensure all exceptions are caught in Image ctor
return unique_ptr from get_image_reader
move std.hpp include into image_reader.hpp (g++)
image-readers: move concrete implementations headers into platform/default
Update README.md
Remove rotation constraints

This should be an iOS SDK option, not hard coded into
low-level renderering.

Fixes #458
Fix OS X build
Merge branch 'master' into image-readers
image_readers: premultiply alpha
Remove other rotation constraint (fixes #463)
Pass FileSource by reference
Eliminate Map::getFileSource
Use noncopyable
Merge branch 'master' into image-readers