add jpeg lib
add image_reader abstraction layer to hanfle multiple image formats
(work-in-progress)
Merge branch 'master' into image-readers
Merge branch 'master' into image-readers
add cflags for linux builds
move image_reader to mbgl-linux
fix tests for linux (todo - find a better way to run 'linux' tests on 'osx')
cleanups
Merge branch 'master' into image-readers
fix linux build
remove unused pragmas
simplify factory class to not rely on singleton + cleanups
remove travis default installed libs which might conflict
Revert "remove travis default installed libs which might conflict"

This reverts commit 23d3588f500f79cc5f532ae7aaf687aaacd6568d.
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)
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++)