Update OSX install instructions in INSTALL.md

Removed unnecessary steps, added support for A/V libs in core and missing dependencies.
Some extra fixes.
Make rtp & codec actions thread-safe
Fixed conflicts
This might be causing problems
Merge branch 'mannol1-master'
Fixed bug when loading friends list already containing an added friend.
Check if call is active after getting mutex handle
The width and height set during the video encoder initialization is
now described as the maximum width and height of images.

This is to work around what appears to be a bug in libvpx where the
resolution of the stream can be decreased but increasing it above
its originally set value introduces memory corruption.
Merge branch 'patch-5' of https://github.com/mouseym/ProjectTox-Core
Merge branch 'master' of https://github.com/mannol1/toxcore
delete groupchat when join fails
Fixed msi_session being used after being freed.
Merge branch 'del-failed-groupchats' of https://github.com/JFreegman/ProjectTox-Core
Update ping_array.c

Change calloc call on line 138 from empty_array->entries = calloc(size * sizeof(Ping_Array_Entry), 1);, to empty_array->entries = calloc(size, sizeof(Ping_Array_Entry));
Merge branch 'kyconny-patch-1'
Typo fix.
Update for OS X build instruction
Merge branch 'toxxxer-patch-1'
use callbacks for a/v