Add maxResults param

[rasa]

This will speed up processing, as the code was defaulting to retrieving 100 messages. Now, it will retrieve 500 messages by default, unless the user specifies a lower amount via:

gmail = Gmail()
gmail.maxResults = 10
messages = gmail.get_messages(query='example')

[OttomanZ]

@jeremyephron Hi, can you take a look into adding this into the system, I believe that this is a must have for many applications that rely on consistently getting inbox results, and if we are fetch all of them it slows us down.