A/V modifications

Hopefully last _major_ modification i'm gonna make before alpha. It lacks Makefile.inc though.

CHANGES:
    - All msi stuff is now in toxmsi.*
    - All rtp stuff is now in toxrtp.*
    - All media stuff is still in toxmedia.*
    - Everything is now in toxav
    - It is still compiled as 3 differend libs
    - Added event handler in core, which is used by all three libs
    - Msi and rtp both support encryption now
    - Msi is now more core-friendly which means it's easier to use along with core
    - NO MORE(that much) UNDERSCORES
    - Changed naming
    - _Pretty_ code. wow
    - Changed terminology in INSTALL.md. It's only fair since we do use a lot of GNU tools
    - Commented everything. It's ready to merge with doc tools and stuff
    - Broke the phone. But at least it now works by adding friend with their id and then handling them with that id