This is an incomplete fix, but a reasonable one for now.

Basically, mpl has its own datetime system, where it uses floating
point versions of the Gregorian calendar days. My attempt to use
ISO formatted date-time strings failed. Therefore, I'm actually
manually formatting the time-string based on epoch time after
converting from mpl_date -> epoch time. Whew!
Add support for barcharts with x-axis date data.
Assume utc time for all dates. This is strictly correct (see below).

Matplotlib's docs state:
'objects are converted to floating point numbers which represent time
in days since 0001-01-01 UTC, plus 1.'

We convert to epoch time with mpl, so it's safe to assume we've got
utc time!

http://matplotlib.org/api/dates_api.html#module-matplotlib.dates
Merge branch 'master' into andrew-fix-mpl_dates
Merge pull request #122 from plotly/andrew-fix-mpl_dates

This is an incomplete fix, but a reasonable one for now.