Merge pull request #3 from dnet/master

raising the bar
[enh] added new chaining DH pfs mode
[mod] (c)
[doc] chaining dh walkthrough
[enh] added persistence of ctx
[mod] __repr__ instead of str()
[mod] added out of band sending test
[doc] specification for chainging dh pfs mode
[doc] update spec
[doc] update spec
[doc] update spec
[mod] renamed walkthrough
[doc] update spec
[doc] update spec
[doc] update spec
[doc] update spec
[mod] added .gitignore
[enh] caching of context identities for loading/saving the context
[doc] typo
[enh] dh chaining mode replaced naive chaining mode