Added full parse support for microseconds.

In response to [this](https://github.com/crsmithdev/arrow/issues/37) issue.

Microseconds can now be specified to full resolution (6 places).

Regular expressions and hard-coded if-else token testing have been entered to match style.

Please note that I have not actually run the unit tests because I have python 3 and did not have time at the moment to install a version of Chai that supports it. I *have* tested parsing by hand, though, and it appears to work.