Removed api.header

[mark-adams]

The header function isn't a public part of the API, is not covered by any tests, and really doesn't seem to serve a practical purpose so I propose removing it.

[coveralls]

Coverage increased (+1.66%) to 65.14% when pulling 77b806f on mark-adams:remove-header into 9715466 on jpadilla:master.

[coveralls]

Coverage increased (+1.66%) to 65.14% when pulling 77b806f on mark-adams:remove-header into 9715466 on jpadilla:master.

[jpadilla]

@mark-adams yeah let's get rid of it. Thanks!

[skion]

Hmm, what is now the suggested way to access the unprotected header before calling decode()?

My use case is that I need access to a kid header which is in turn used as a parameter to decode.

[mark-adams]

@skion See #155