Implement HttpSig

[bblfish]

Implement the HttpSig authentication scheme.

[bblfish]

Initial implementation of HttpSig was pushed in commits 662c0d5 and f3ed53e using the Java http-signatures-java library that implements Version 00 of "Signing HTTP Messages". As a result I have stuck with Authorization: Signature... header, as at present I just need to test that the architecture of the system works. Note that the question of what the form of the Authorization header is opened in HttpSig, Signature or Solid?.

I am still researching what ontology to use: see Ontology for the keyId document. This is needed to be able to interpret the keyId resource. I may just invent a couple of relations to get going.

[bblfish]

The code for everything up to "Signing Http Messages" has been moved to the httpSig repository and generalised to compile to JS libs too. This has then been pulled back in here with PR 20.
The code for authentication should perhaps be pulled back out again later to the httpSig repository again to make it widely useable.