sparse module: add a RandomSetter based on a user defined map implementation

as described on the wiki (one map per N column)
Here's some bench results for the 4 currently supported map impl:
std::map =>             18.3385 (581 MB)
gnu::hash_map =>        6.52574 (555 MB)
google::dense =>        2.87982 (315 MB)
google::sparse =>       15.7441 (165 MB)
This is the time is second (and memory consumption) to insert/lookup
10 million of coeffs with random coords inside a 10000^2 matrix,
with one map per packet of 64 columns => google::dense really rocks !
Note I use for the key value the index of the column in the packet (between 0 and 63)
times the number of rows and I used the default hash function.... so maybe there is
room for improvement here....

Eigen/Sparse

@@ -67,6 +67,7 @@ namespace Eigen {
 #include "src/Sparse/SparseMatrixBase.h"
 #include "src/Sparse/SparseArray.h"
 #include "src/Sparse/AmbiVector.h"
+#include "src/Sparse/RandomSetter.h"
 #include "src/Sparse/SparseBlock.h"
 #include "src/Sparse/SparseMatrix.h"
 #include "src/Sparse/HashMatrix.h"