Merge pull request #337 from bvssvni/master

Upgraded to latest Rust-Empty
Added links to docs for sdl2_ttf and cgmath

Closes #340
Merge pull request #341 from leonkunert/master

Added links to docs for sdl2_ttf and cgmath
Added portaudio and SoundStream trait for real-time audio IO. Example included.
Fixed soundstream example header
Merge pull request #345 from mitchmindtree/master

Added PortAudio backend and SoundStream trait for non-blocking, real-time audio IO.
Added condition compilation for PortAudio