don't set the window title on every frame

this is a significant performance penalty, at least on OS X, and drops our frame rate below 60fps on even simple scenes. instead of showing the position of the map in the window, we should display it elsewhere
switch to a newer version of GLFW

refs #679
break out FileSource as an abstract class

add CachingHTTPFileSource implementation
Merge pull request #677 from mapbox/caching-http-file-source

Break out FileSource interface
Merge branch 'master' into android-mason

Conflicts:
	gyp/mbgl-ios.gypi
	gyp/mbgl-linux.gypi
	gyp/mbgl-osx.gypi
	platform/default/caching_http_file_source.cpp