Implement Matrix Addition

Main changes
============

* Implemet the addition of two dense Matrices.
* Now the `MatrixBase` is derived from Basic, allowing Matrix classes to be
  used as any other CSymPy objects.
* Change the access modifier of `row_` and `col_`of `MatrixBase` from `private`
  to `protected`.
* Declare the functions used to add Matrics as `friend` functions.

TODO
====

* Improve `DenseMatrix::compare`.
* Add more tests.