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)
I believe that it would be cleaner if the constructor looked something like:
```
def __new__(cls, index, name=None):
    if name is None:
        name = {0: 'i', 1: 'j', 2: 'k'}[index]
```