Add support for general matrix functions.

This does the job but it is only a first version. Further plans:
improved docs, more tests, improve code by refactoring, add convenience
functions for sine, cosine, sinh, cosh, and (eventually) add the matrix
logarithm.

unsupported/Eigen/src/MatrixFunctions/MatrixExponential.h

@@ -72,7 +72,7 @@
  *     \end{array} \right]. \f]
  * This corresponds to a rotation of \f$ \frac14\pi \f$ radians around
  * the z-axis.
-
+ *
  * \include MatrixExponential.cpp
  * Output: \verbinclude MatrixExponential.out
  *