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)
IMHO it shouldn't be like this.

```
In [4]: from sympy.vector import *

In [5]: i = BaseVector('i', 0)
In [7]: i.is_Vector
Out[7]: True

In [8]: i.is_Vector = False

In [9]: i.is_Vector
Out[9]: False
```