Merge pull request #373 from bvssvni/master

Made Piston compile without PortAudio settings
Merge pull request #375 from bvssvni/master

Removed debug comments
Merge pull request #377 from bvssvni/master

Added example to `GameIterator` docs
Merge pull request #379 from bvssvni/master

Added creation of graphics context to `GameIterator` docs
Merge pull request #381 from bvssvni/master

Updated `GameIterator` docs with 'ref' and 'mut'
Removed constructor from `GameWindow` trait

Closes https://github.com/PistonDevelopers/piston/issues/383