ENH: Simple MI metric evaluation

This is a simple program for mattes mutual information metric
evaluation over two input images and a BSpline transform.

If there is no input BSpline transform, the transform will be
set to Identity as the default.

The Usage:
./PerformTestMMI Image1 Image2 [BSplineTransformFile]
                 [NumberOfHistogramBins] [NumberOfSamples]
Merge pull request #82 from aghayoor/MIMetricEvalOnBSplineTrans

ENH: Simple MI metric evaluation