Added a Hessenberg decomposition class for both real and complex matrices.
This is the first step towards a non-selfadjoint eigen solver.
Notes:
 - We might consider merging Tridiagonalization and Hessenberg toghether ?
 - Or we could factorize some code into a Householder class (could also be shared with QR)
diff --git a/test/nomalloc.cpp b/test/nomalloc.cpp
index 6d40222..d6c566b 100644
--- a/test/nomalloc.cpp
+++ b/test/nomalloc.cpp
@@ -22,7 +22,9 @@
 // License and a copy of the GNU General Public License along with
 // Eigen. If not, see <http://www.gnu.org/licenses/>.
 
+// this hack is needed to make this file compiles with -pedantic (gcc)
 #define throw(X)
+// discard vectorization since operator new is not called in that case
 #define EIGEN_DONT_VECTORIZE 1
 #include "main.h"