initial implementation to display displacement fields as quiver plots

Currently this implementation only works with 2D and does not work
with interactive versions of my show, due to need to access the
current matplotlib axes before it is shown.
fix extraneous statement in myshowDisplacement function
ENH: First test is added

This test needs three input argument: fixed image, moving image and the
baseline image all from the BRAINSTools test data.

It runs 4 stages of registration, and itkTestDriver is used to compare
the output moving resampled image to the baseline image.

Many function definitions that are common between different tests are
put in one file (TestFilesCommonCode.ipynb), and this file is run at the
beginning of each test using ipython magic functions.
Updating with additional example more typical of registration

The prior example was erroneous with the rotation happening at the
origin. A new test case there the fixed image has been translation
some, into a large image is a better test case. With this setup we
experiment with a composite transform there the first is a complete
solution and where it's only a partial, and observe the effects of
rotation about the center. This clearly demonstrates that the center
parameter for the transforms is in the fixed image's domain, not in a
intermediate range in the composite transform.
Merge pull request #1 from aghayoor/notebookTests

ENH: First test is added
Merge pull request #2 from blowekamp/UpdateCompositeTransformCenter

Updating with additional example more typical of registration
ENH: Updated the Readme.md
ENH: Added infrastructure for git hooks and remote data

This uses the same infrastructure as the SimpleITK-Notebooks
for maintaining outputs.
ENH: Added data sources to repository.
ENH: Cleared outputs from notebooks.
COMP: Preparing for merging, removed all outputs.