Allow passing query parameters as a params kwarg to underlying Requests

It would be nice if the query params could be specified as a kwarg to the `get`, `post`, et al methods as the [underlying Requests library does](http://docs.python-requests.org/en/master/user/quickstart/#passing-parameters-in-urls). 

Ex:
```python
wcapi = API(...)
params = {
    'status': 'cancelled',
    'per_page': 25,
    'page': 1
}
r = wcapi.get('orders', params=params)
```