This commit adds lossless compression capability to the experimental
branch. The lossless experiment can be enabled using --enable-lossless
in configure. When the experiment is enabled, the encoder will use
lossless compression mode by command line option --lossless, and the
decoder automatically recognizes a losslessly encoded clip and decodes
accordingly.

To achieve the lossless coding, this commit has changed the following:
    1. To encode at lossless mode, encoder forces the use of unit
quantizer, i.e, Q 0, where effective quantization is 1. Encoder also
disables the usage of 8x8 transform and allows only 4x4 transform;
    2. At Q 0, the first order 4x4  DCT/IDCT have been switched over
to a pair of forward and inverse Walsh-Hadamard Transform
(http://goo.gl/EIsfy),  with proper scaling applied to match the range
of the original 4x4 DCT/IDCT pair;
    3. At Q 0, the second order remains to use the previous
walsh-hadamard transform pair. However, to maintain the revers...