Update Grant and Token Getters and Setters

[flippmoke]

This is primarily an update to change the way that the grant and token getters and setters work. Rather then utilizing the underlying Requests class from oauthlib, the parameters required are passed individually. This makes it more clear what should be saved directly in the setters.

[flippmoke]

Just noticed an issue where I screwed up the merge. Going to correct the issue then resubmit.

[lepture]

Why do you want to change usergetter to usernamegetter?

[lepture]

https://github.com/lepture/flask-oauthlib/blob/master/CONTRIBUTING.rst#git-help

don't merge any code yourself

[flippmoke]

I wanted to change the usergetter to usernamegetter because it was a better way of describing the password credentials only request, which is essentially looking for a username and password. If you can think of a better name, I am open to changing it.

The usergetter I added should be used to pull the information on the currently logged in user. This is useful for creating a user object when using the authorization handler. The use of the usergetter name here makes the most sense to me, and is why I changed the previous usergettter to usernamegetter

[lepture]

@flippmoke I don't like the idea. Thanks anyway, version 0.2.0 is nearly out. #12