change most type checks to `numbers.Number`

Except for `width` and `height`, the vega docs state all numeric properties to be of
type `Number`. This can be mirrored in Python using the `numbers` module.

Pros: 
    1. Any numeric value inherited from `int`, `float` or any of the `numbers.Number` base classes will pass the `isinstance` checks.
Cons: 
    1. `numbers.Complex` may not be a desirable numeric type to allow.
    2. The `numbers` module was added in Python 2.6 thus excluding earlier versions.

I'm fully open to changing all occurences of `Number` to (`int`, `float`) but I like that it matches the vega docs.