libuv handles must be deallocated asynchronously

Fixes #659
Remove unused
Prefer stdlib to libuv (fixes #688)
[performance] remove use of snprintf in hot path; it's pretty slow on iOS
remove use of std::to_string in favor of boost::lexical_cast

std::to_string calls sprintf() internally, which is really slow
Update mapbox-gl-cocoa
Add virtual destructor for FileSource
switch to plain file streams rather than JSON
make most headers private
remove usage of relative paths
fix build
Merge pull request #622 from mapbox/private-headers

make most headers private
disable uv_thread_self() == map_thread asserts, ref #623
add locking and logging
fix merge conflits