Provides a read-only interface for reading byte and bit-sized data from
an underlying buffer in network/big-endian order. Also provides a method
for reading exponential golomb encoded values, which will be useful in
H.264 packet parsing (separate CL).
BUG=
R=pthatcher@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/49719004
Cr-Commit-Position: refs/heads/master@{#9046}
