Further improve speed of mime base64 encoding routines.
Base 64 decode is a little faster than libb64 on my AMD FX6100, and
a very slightly slower (a few percent) for encoding.  The speeds of
encoding and decoding are both approximately 4x faster than before
these changes.

Also optamize util.gettimeofday() and util.gettimemonotonic().
Merge pull request #122 from jsolman/master

Further improve speed of mime base64 encoding routines.