Enable %Y-%b-%d %H:%M:%S.%f timestamp format (used in boost)

Good night, Graham.
I've made a small change to your wonderful library, it enables parsing %Y-%b-%d %H:%M:%S.%f timestamps, de-facto default format for Boost C++ applications: https://svn.boost.org/trac/boost/ticket/8839.
This format does not seem to be standard one yet it is reasonably common.
Two additional tests are added. None of the tests look broken.
Please consider accepting my commit. Thank you.