Added sodium.Key.SignKey.fromSeed(seed, encoding)

Summary more or less speaks for itself... IMO the seed is the key, I want to use this to secure REST APIs, and distributing a 44 char base64 api-key is more, fun than a 88 char base64 api-key... :)

I also took the liberty of adding a test with hardcoded key, so that we test that keys can be loaded..
Along with a test for fromSeed()... I used the javascript port of tweetnacl to generated the hardcoded keys and seed. So that should show some interoperability. Sanity checks are always good :)

By the way, any plans to merge detached signing from #11? If the fork is merge worthy can we at least add detached signing? Personally I hate to cut it out of the buffer...