donation api #16

Based on https://roboticdogs.actionkit.com/docs/manual/api/rest/donationpush.html
make a file actionkit/api/donate.py (similar to action.py that extends base.ActionKitApi
https://github.com/MoveOnOrg/python-actionkit/blob/public-master/actionkit/api/action.py#L7)

make a method in that file that is basically:
def donate(self, user_id, donation_page_name, amount):
and sends the right post and gives the right details back (like success or if there was an error -- e.g. in charging)

(then you can use it similarly to e.g. eventroller project here:
https://github.com/MoveOnOrg/eventroller/blob/master/event_exim/connectors/actionkit_api.py#L9-L10
https://github.com/MoveOnOrg/eventroller/blob/master/event_exim/connectors/actionkit_api.py#L32-L34
https://github.com/MoveOnOrg/eventroller/blob/master/event_exim/connectors/actionkit_api.py#L151-L156
https://github.com/MoveOnOrg/eventroller/blob/master/event_exim/connectors/actionkit_api.py#L337