Linux support

[davejm]

Adds support for running and building for Linux 64bit.

Note that boilerplate comments are set up in the gulpfile to copy a custom build ffmpeg library for Linux into the dist, however, I don't have this at the moment so sound doesn't work. Making it work would be a matter of getting the custom libffmpegsumo.so library built for Linux 64bit with the relevant codec support, putting it in ./lib and uncommenting the code in gulpfile for copying the linux library.

[davejm]

@antiboredom do you have the ffmpeg lib with mp3 codecs etc. for linux (and nw.js 12.3)?

[jedahan]

After reading https://nodejs.org/api/process.html#process_process_platform , I think I most prefer the simplicity of

var isWin = process.platform === 'win32';
var isMac = process.platform === 'darwin';
var isLinux = process.platform === 'linux';

Honestly this is nitpicking at this point, and would love to see this merged sooner than later :)

[davejm]

@jedahan yes that is cleaner. I had looked at that api page as well but assumed from the regexes that the method could return -32 and -64 variants but it seems not so I'll change it.