Add a version check for optional matplotlib dependency.

Fixes Issue #98. 
Yah, but it would've been more helpful I think if we received an error when
we tried to use `plot_mpl` rather than a warning on import. We ran a
script, so we saw that huge traceback, and probably would've missed the
import warning.


On Mon, Jul 28, 2014 at 7:27 PM, Andrew <notifications@github.com> wrote:

> In plotly/tools.py:
>
> >  except ImportError:
> >      _matplotlylib_imported = False
> > +else:
>
> Right, but in that case the user *wanted* to use matplotlib
> functionality, right? Therefore warning was a good thing here?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/plotly/python-api/pull/99/files#r15497951>.
>