Allow setting audience and issuer

[cancan101]

Right now there is no way to pass in the audience to validate which causes the following to fail:
https://github.com/jpadilla/pyjwt/blob/5f9bc05803508b844d3fd9c61cd5210af06eecd8/jwt/api.py#L169

closes #79
closes #81

[jpadilla]

@cancan101 this is good, thanks! Could you update any docs with this new setting? Also, might be a good idea to include a simple test since this is adding a new "feature".

[cancan101]

How do I run tests locally? This ./runtests.py --fast fails with:

E       OperationalError: no such table: auth_user

[jpadilla]

@cancan101 that's strange, still running into issues running tests?

[cancan101]

This command still has the same error: ./runtests.py --fast but the tox commands work fine: tox -e py27-django1.7-drf3.0.0

[jpadilla]

@cancan101 hmm weird, works fine here. I'll look into what might be going on. The only things I think is missing is the test case for this, everything else looks fine.

[cancan101]

Suggestion where to put the test? https://github.com/GetBlimp/django-rest-framework-jwt/blob/master/tests/test_utils.py seems like the right place.

How do I test changing the settings?

[jpadilla]

Yeah that seems right.

There's an example here..

[cancan101]

Tests uploaded.

[jpadilla]

(do not check iss on JWT)

Do you mean that if JWT_ISSUER is None and iss is present on the JWT it won't be checked? This bit could be a little more clearer.

[cancan101]

I was just trying to interpret this code which checks the iss on the token only if issuer is not None:

    if issuer is not None:
        if payload.get('iss') != issuer:
            raise InvalidIssuerError('Invalid issuer')

[jpadilla]

@cancan101 thanks!