Don't read an uint8_t array as an uint32_t

This makes the code work properly on big endian.

The MC case is similar to how it's done in the encoder.

Neither of these should have any significant performance
impact.
Fix shifting tricks in deblocking for big endian

The code interprets an array of 4 uint8_t values as one uint32_t
and does shifts on the value. The same optimization can be
kept in big endian as well, but the shift has to be done in the
other direction.

This code could be made truly independent of endianness, but
that could cause some minimal performance degradaion, at least
in theory.

This makes "make test" pass on big endian, assuming that
WORDS_BIGENDIAN is defined while building.
Merge pull request #256 from mstorsjo/bigendian

Make the library work on big endian