ENH: Generalize the legacy PerformTestMMI

PerformTestMMI was a simple program to evaluation Mattes metric value
between two images after warping the second image if a BSpline transform
is defined.
The above program is expanded to work with different metric type that is
defined by a "--metricType" flag. Currently MMI (mattes mutual
information) and MSE (mean square error) metrics are supported.
Merge pull request #140 from aghayoor/performMetricTest

ENH: Generalize the legacy PerformTestMMI