Merge pull request #91 from Kimundi/master

Update submodules
Implemented proper dependencies for Makefile
Merge pull request #92 from kvark/make

Proper dependencies for Makefile
Make version parsing more lenient and improve documentation
Add some assorted documentation
Formatting fixes
Make Int attribute matching clearer by splitting into two cases
Fix LD_LIBRARY_PATH on travis
Merge pull request #93 from bjz/cleanups

Assorted documentation and formatting cleanups
Introduced ResourceCache
Added Request/Reply tokens
Revamped render<->device messaging
Made glfw window creation use a proper builder pattern
Merge pull request #94 from Kimundi/window_builder

Made glfw window creation use a proper builder pattern
Make GlBackEnd::check's failure message more informative.
Merge pull request #96 from bjz/check

Make GlBackEnd::check's failure message more informative.
Implemented MaybeLoaded<> and actual async resource receiving on the render side
Switched all resources to async. Removed the future from Renderer.
Moved resource cache into a separate module
Added more comments to resource-related code