Merge pull request #336 from Coeuvre/master

Upgraded to latest gl-rs
Upgraded to latest Rust-Empty
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
Merge pull request #346 from bvssvni/master

Added condition compilation for PortAudio
Removed `AudioSDL2` from `GameIterator`

Closes https://github.com/PistonDevelopers/piston/issues/347
Merge pull request #348 from bvssvni/master

Removed `AudioSDL2` from `GameIterator`
Removed `InteractiveEvent`, added `to_sendable` to `GameEvent`
Merge pull request #349 from bvssvni/master

Removed `InteractiveEvent`, added `to_sendable` to `GameEvent`
Making library compile with port audio

`make COMPILER_FLAGS="-O --cfg port_audio”`
Merge pull request #350 from bvssvni/master

Making library compile with port audio
Fixed sound stream example partially
Merge pull request #351 from bvssvni/master

Fixed sound stream example partially
Fixed audio_iter example
Merge pull request #352 from bvssvni/master

Fixed audio_iter example