BUG: Fix matplotlib compatibility

BayesPy should now work with MPL<1.4.0, MPL==1.4.0 and MPL>1.4.0.
In addition, plotting should work on scripts, Python shells and
IPython shells.
BUG: Fix missing moments in Gamma->Wishart converter node
DEMO: Fix matplotlib issues in demos
BUG: Fix minor indexing bugs in plot module
ENH: Add subplotting method to plot module
DOC: Fix doctest errors
MAINT: Add requirements.txt for building docs