install mason automatically if it isn't present, and always use the install in ~/.mason
update readme
[skip ci]
make config.gypi files dependend on the configure script
[skip ci]
add information to readme about clearing the build cache
[skip ci]
restructure gyp files
Merge remote-tracking branch 'origin/headless-display' into gyp-restructuring

Conflicts:
	Makefile
	ios/mapbox-gl-cocoa
make tests work