Add --exclude-monotonic command line option.

--exclude-monotonic prevents faketime from overriding
the clock with id CLOCK_MONOTONIC when using clock_gettime.

Add DONT_FAKE_MONOTONIC env variable to libfaketime that
has the same effect.

Add functional test for DONT_FAKE_MONOTONIC support.
Merge pull request #49 from joyent/dont-fake-monotonic-clocks

Add --exclude-monotonic command line option.