Added aom_write_tree_cdf() and aom_read_tree_cdf() function calls to
 bitwriter.h and bitreader.h respectively.
These calls take a multisymbol CDF and an index and directly encode the
 symbol using the enabled entropy coder.
Currently only the daala entropy encoder supports this (enabled with
 --enable-daala_ec) and a compile error is thrown otherwise.

Change-Id: I2fa1e87af4352c94384e0cfdbfd170ac99cf3705