Add ability to specify other headers when generating a token

[petedmarsh]

The RFC7519 spec defines the following headers:

typ
cty
alg

alg and typ are set appropriately by generate_token, however there
hasn't been a way to specify cty or any other headers defined by
later standards (i.e. by JWE and JWS).

This adds a new optional parameter to generate_token: other_headers.
If specified the key/values will be included with the headers in
the token. To avoid misue, if other_headers specifies 'typ' or
'alg' a ValueError will be raised.

[petedmarsh]

I wasn't able to get the tests running locally and I'm unfamiliar with the test framework, apologies for not adding tests. I recently had to generate a token with a kid in the header and it would be great to be able to do so with generate_token.

[davedoesdev]

@petedmarsh appreciate this. I'll merge it and add tests.
(I chose PyVows way back when but it's been unmaintained for a while now so adding tests isn't as simple as it used to be).

[davedoesdev]

@petedmarsh now available as version 3.1.0.
Thanks again.