Make Revocation RFC compliant

[jbspeakr]

The current POST /revocations is not compliant to the corresponding RFC 7009.

Also it's currently unknown why there are multiple revocation types. This implicit knowledge needs to get clarified and made explicit with proper documentation. Alternatively, the implementation needs to get fixed (which will make breaking API changes unavoidable).

[gargravarr]

Documentation regarding revocation types can be found here http://planb.readthedocs.io/en/latest/revocations.html.

[jbspeakr]

@gargravarr thx for the link. unfortunately it's not clear why there are multiple revocation types and whether or not there are Zalando-internal or external requirements for this. The spec certainly doesn't know them.