Basic vector code, without a coordinate system class

All calculations essentially occur in the same frame.
i, j, k and x, y, z are imported as base vectors and base scalars by default on importing sympy.vector.
delop is a Del operator instance.

Example:

>>> v = 3*i - j
>>> delop ^ v
0

(delop ^ v represents curl of v)
Thats just for SymPy notation consistency. 
The problem is I havent <code>is_Vector = False</code> to Expr.
Otherwise, we just assume that users don't play around with functional attributes. Otherwise, Python's all-public OOP style will complicate matters in any scenario. 
Aaron and I had discussed this earlier, the 'is_xxx' notation should probably be phased out.