Added histogram min and adjusted unit tests.

[kkumler]

Implementing #1219 (thanks for adding the issue -- I saw it there and figuring I'd write something, after having asked support earlier).

tests/test_bucket_aggregator.py tests/test_dogstatsd.py tested clean. I could not execute the entire test suite locally yet.

[LeoCavaille]

Looking good @kkumler thank you so much for taking the time to implement this! 👍
It looks like this modification breaks other test cases that are depending on histograms, for instance tests.test_go_expvar.TestGoExpVar (see the Travis build with flavor default), would you mind fixing those test cases? Some of the Travis builds might be failing because of flaky tests or deps, but we are working on improving that.

[kkumler]

@LeoCavaille I fixed the one I missed there.

[LeoCavaille]

Thanks @kkumler again, see #1303 for the final merge with your code and how to use the new behavior.