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
Remove invalid map->terminate()

Map has already been destroyed by this point, so the
call is invalid. The work that terminate() would do
has already been done.
Move setReachability to CachingHTTPFileSource
Add CachingHTTPFileSource::clearLoop()

So loop state can be cleaned up in the appropriate thread.
This is a hack; loop needs to be externalized from Map.

Fixes #686
get/setBase may be called from main thread
Create GlyphStore up front
Change Map::stop to use std::function<void()>

Fixes #685
Move setAccessToken to CachingHTTPFileSource
disable uv_thread_self() asserts, ref #623