Remove dependecy toward libsndfile on all OS

This implements #604. It's a rebase of `feature/no_libsndfile_osx` onto master (which includes `feature/no_libsndfile`) so be extra careful when proofreading it. Hopefully I haven't made any mistake but there were a lot of conflicts so... 

What's done & what's to be done:

 * [ ] Android (all libs **missing**)
 * [x] iOS (all libs included)
 * [x] Linux (no binary to be provided)
 * [x] OS X (all libs included)
 * [ ] Window (FLAC build for MinGW **missing**)

It also includes the base step toward #620.
All the ```loadFromFile``` functions in SFML already contain Android specific code, to use the dedicated ResourceStream class. Can't all this stuff be mixed nicely?