Allow for client_id and client_secrets in init params

I need to use the FuelSDK for several clients (Apps). This allows me to iterate over those separate App IDs. The use-case looks something like the following:

```
from ConfigParser import ConfigParser
import os
import FuelSDK

config = ConfigParser()
full_path = os.path.join(os.path.dirname(__file__), 'settings.ini')
config.read(full_path)
for s in config.sections():
    params = dict(config.items(s))
    client = FuelSDK.ET_Client(params = params)
```

I've emulated the existing check for JWT params.