implemented /sign with hawk credentials

uses the signToken to derive the hawk
tokenId and reqHMACkey as specified:
https://wiki.mozilla.org/Identity/AttachedServices/KeyServerProtocol#Signing_Certificates
removed extra JSON stringify/parse
added /sign tests with invalid data

the payload validation test will fail until hapi is fixed.
see https://github.com/spumko/hapi/pull/949
Merge pull request #47 from dannycoates/signCert

implemented /sign with hawk credentials