Change AppDirs links in docs to always point to latest version [Close #284]
Update installation documentation [Close #288]

* Improve Linux instructions
* Add Mac instructions
* Add ffmpeg links and instructions
Change Sphinx config to insert code dir at beginning of sys.path

This makes sure that the local dev code is used before installed versions of gmusicapi when building docs.
Merge pull request #292 from thebigmunch/issue284

Change AppDirs links in docs to always point to latest version
Merge pull request #291 from thebigmunch/sphinxconf

Change Sphinx config to insert code dir at beginning of sys.path
Add ffmpeg to misc encoder references [#288]
Merge pull request #293 from thebigmunch/issue288

Add ffmpeg to documentation
Fix mp3 encoding support detection [#269]
Merge pull request #294 from thebigmunch/issue269

Fix mp3 encoding support detection