Use requests.Session for TCP persistence

The requests.Session object provides a persistent TCP connection.  Use
one of those for each HabitRPG instance, rather than creating a new TCP
connection for every API request.
Add support for URL parameters on requests

Some API methods require arguments to be passed as encoded URL
parameters.  Allow those parameters to be passed into the api_request
methods.
Add Group and Party classes for sending chats

Add a Group class, with a method for sending a chat message to the
group.  Also add a Party class, which is a subclass of Group, but which
doesn't require a group ID to be specified.
Handle HTTP 204 return codes

The HTTP 204 status code means "No Content", and is sent by the HabitRPG
API for at least some API responses.  Decoding the content of these
responses fails, because there isn't any, so don't attempt to decode
them.
Add a ChatMessage class

This is the very basic outline of a class to be instantiated for each
message in a chat log.  Currently it only supports a delete() method,
and in particular doesn't yet support being created from an API
response.