libobs: Add audio control functions

This adds a new library of audio control functions mainly for the use in
GUIS. For now it includes an implementation of a software fader that can
be attached to sources in order to easily control the volume.
The fader can translate between fader-position, volume in dB and
multiplier with a configurable mapping function.
Currently only a cubic mapping (mul = fader_pos ^ 3) is included, but
different mappings can easily be added.

Due to libobs saving/restoring the source volume from the multiplier,
the volume levels for existing source will stay the same, and live
changing of the mapping will work without changing the source volume.
obs: Use new fader from libobs for volume control

This replaces the code for volume control with a newer version that
uses the new fader implementation in order to control the fader/slider
position and source volume.
The volume label will now indicate the gain in dB instead of percent,
mainly because it looks cool and is easy to do.
Due to libobs saving the multiplier option for the source instead of
the slider/fader positon, existing volume levels will (mostly) stay
the same with only the slider beeing at a different position.
This is of course within the resolution of the slider (100 steps).
Clean up CONTRIBUTING file

There's a little too much junk in the CONTRIBUTING file.  I feel like
it's unnecessary to say some of the things that were being said in
there.  It really needs to be to the point
Merge pull request #304 from fryshorts/obs-audio-controls

Add advanced audio controls
libobs: Add process piping functions

This adds functions for piping a command line program's stdin or stdout.
Note however that this is unidirectional only.

This will be especially useful later on when implementing MP4 output,
because MP4 output has to be piped to prevent unexpected program
termination from corrupting the file.
libobs: Fix windows warnings with C library macro

The macro INFINITY apparently will trigger a warning with microsoft's
compiler despite being a C standard library macro.
libobs: Fix double -> float conversion warning
libobs: Fix warnings assigning values to vec3

Because a vec3 structure can contain a __m128 variable and not the
expected three floats x, y, and z, you must use vec3_set when
setting a value for a vec3 structure to ensure that it uses the proper
intrinsics internally if necessary.
libobs: Fix microsoft compiler issue

In older versions of visual studio 2013 microsoft's WORTHLESS C compiler
has a bug where it will, almost at random, not be able to handle having
variables declared in the middle of a function and give the warning:
"illegal use of this type as an expression".  It was fixed in recent
VS2013 updates, but I'm not about to force everyone to update to it.