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/doc/examples/MatrixFunction.cpp

@@ -0,0 +1,23 @@
+#include <unsupported/Eigen/MatrixFunctions>
+
+using namespace Eigen;
+
+std::complex<double> expfn(std::complex<double> x, int)
+{
+  return std::exp(x);
+}
+
+int main()
+{
+  const double pi = std::acos(-1.0);
+
+  MatrixXd A(3,3);
+  A << 0,    -pi/4, 0,
+       pi/4, 0,     0,
+       0,    0,     0;
+  std::cout << "The matrix A is:\n" << A << "\n\n";
+
+  MatrixXd B;
+  ei_matrix_function(A, expfn, &B);
+  std::cout << "The matrix exponential of A is:\n" << B << "\n\n";
+}