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 think it's like `class VectorZero(with_metaclass(Singleton, Vector))`

In SymPy, `with_metaclass` is a trick to keep compatibility with both Python 2 and 3 (metaclasses are declared in a different manner in Python 3).