Add more flexible and complete verification options

[mark-adams]

I was thinking that it might be useful for us to implement more flexible verification options.

I propose something like this:

def decode(token, secret, options=None)

where options is a dict that looks something like this:

options = {
   'verify_signature': True,
   'verify_exp': True,
   'verify_nbf': True,
   'verify_iat': True,
   'verify_aud`: True
}

This is similar to what ruby-jwt does
We could make it where options could be specified globally (at the PyJWT object level) or as an override argument to decode()

[jpadilla]

@mark-adams yeah I can see how this can be useful. For example that #129 use case could just set verify_aud to False.

[mark-adams]

I'll take a look

[michaeldavis-wf]

@mark-adams I know timelines are to be taken with a grain of salt, but do you have a guesstimate on how long 1.1.0 might be? I would love to be able to pull in the changes from this PR.

If it is going to be too long, would it be possible to cut a 1.0.2 release with the current master?

[mark-adams]

Moved this convo to #137