Bug with Matplotliv 1.4.0 is fixed

Fix ticket:672

Bug with Matplotlib version 1.4.0 is fixed. the bug was caused because
the default value of vmin and vmax is None internally to
Matplotlib. Some code in matplotlib tries a numpy operation on these
None values. A simple fix is to pass a norm instance to the
ColorbarBase class when it is called in FiPy. This may be an internal
bug to Matplotlib.

The fix works was tried with both Matplotlib version 1.4.0 and 1.3.1.
Fix the "limits" argument to MPL 2D grid viewer

Fix #421

The "limits" argument was not being used correctly in
Matplotlib2DGridViewer internals. The min, max axes limits were never
set. The fix now set the axes limits correctly.
Allow for 2 periodic axes in 3d

Added subclasses of _BasePeriodicGrid3D to describe two dimensions of
periodicity.

re #671

Signed-off-by: Daniel Wheeler <daniel.wheeler2@gmail.com>
Merge branch 'issue415-matplotlib-1_4_0-error' of github.com:wd15/fipy into wd15-issue415-matplotlib-1_4_0-error
Merge branch 'wd15-issue415-matplotlib-1_4_0-error' into develop
Force MatplotlibViewer to display

Matplotlib 1.4 seems to have changed when it shows things when not in
interactive mode. `block=False` prevents the viewer from grabbing focus.

Addresses issue427
Merge branch 'issue421-Matplotlib2DGridViewer_limits' of github.com:wd15/fipy into wd15-issue421-Matplotlib2DGridViewer_limits
Merge branch 'wd15-issue421-Matplotlib2DGridViewer_limits' into develop
Merge pull request #424 from wd15/issue414-PeriodicGrid3D_axes

Allow for 2 periodic axes in 3d
Merge branch 'issue427-MatplotlibViewer_dont_display' into develop

Fix #427