This commit implements a new rate control system which was ported from
Daala's rate control system (which was based off of Theora's rate
control system) to the encoder. Its aim is to address the weaknesses of
the current rate control system and eventually serve as its replacement.

This is still work in progress. So far this commit only implements
support for single pass encoding for constant quality and bitrate
targeting.
Additional commits which implement two-pass support and the rest to
match the feature set of the current rate control system are coming
soon.

The rate control system hasn't been fully tuned (and will likely need
additional tuning as the encoder's development progresses) and does
contain some "hacks", to convert the quality to a quantizer value
(empirical) and to tweak the quantizer in between golden frames (which
uses some code from the current rate control system as well as code
which attempts to model what that code does), the latter of which is
only in the constant quality codepath.

Bitrate targeting works much better than the current rate control
system's targeting and will actually closely match the rate specified by
the user without the current rate control system's bursty behaviour.

Change-Id: I588fbfd2e80a3d21ce7176903115d6a96ef1700a