Add Number Field

It would be nice to have a Field that can handle all types of Numbers. Floats, integers, (longs in Python 2). Perhaps Decimals, and complex numbers too.

```python
from serde import Model, fields

class Example(Model):
    number = fields.Number()

assert Example(number=5.0) == Example(number=5)
assert Example(number=5 + 0j) == Example(number=5)
```