Toggle platforms to build with grunt option instead of manual toggle in
Gruntfile.js
Remove some obsolete comments
Add support for linux32 and separate win from darwin
Document the availability of linux32
Add option to build all platforms
Merge pull request #482 from luuse/choose_platform

Toggle platforms to build with grunt cli option
ref #54, attempting to add support for linux32
ref #615, got libffmpegsumo.so from @daka90, and the build works