(WIP) Basic Vector framework

Added:

1. Vector core - Vector, VectorAdd, VectorMul, BaseVector
2. BaseScalar
3. Del
4. CoordSysRect
5. Point
6. functions.py - express, matrix_to_vector 
I have already implemented <code>_eval_derivative</code> for Vector. But the routine in <code>diff</code> causes the differentiated vectors, if called form the main function to return something like a <code>Mul</code> in place of <code>VectorMul</code>. This will occur if the user wants to differentiate wrt more than one symbols, I believe. I posted about this on the sympy mailing list, Mathew suggested I take this approach for the special case of Vectors.