deprecate undocumented `matplotlib/mpl.py`

`matplotlib/mpl.py` is a file which used to get imported via ipython's
`%pyplot` interface as module named `mpl`, yet it isn't clear at all
what it's role is.

Given that in our own code, and in the rest of our python neighborhood,
we often do `import matplotlib as mpl`, it's quite a bit confusing to
have something with the `mpl` name within matplotlib not be the
`matplotlib` module itself.

This module was only used in `pylab` interface, where it was simply
imported in one line via `from matplotlib import mpl` and never used
again. With the exception of mpl.finance, all of the modules in mpl.py
were readily available simply via "import matplotlib as mpl".

pylab's mpl.finance functionality is retained for backwards
compatability.
move to the new MatplotlibDeprecationWarning
added deprecation note to API changes
use mplDeprecation convention
Add more verbose removal warning stating version
Merge pull request #1670 from dmcdougall/deprecate_mpl

Deprecate mpl