audio rework

this gets rid of hard-coding of audio mimetypes and bins in various bits of code, and also makes a single audio bin which will work for any(?) audio source.

the new "audio" bin replaces the pulse and autoaudio bins and adds the ability to set the audio src with the "src = xxx" config option in the relevant track in your profile. tested with autoaudiosrc (default), alsasrc and pulsesrc. (fixes #53)

audio mimetypes are now guessed in the same way as video ones (just 'audio/file_ext') or you can specify a mimetype for any track by adding a "mimetype = xxx" in the relevant track your profile. (fixes #78)

this probably needs testing on different hardware to make sure i haven't messed anything up with the mimetype stuff. let me know if i should change anything.