Added sodium.Key.SignKey.fromSeed(seed, encoding)
Ported to latest node 0.11 with nan (Native Abstractions for Node.js)
Detached signatures. Documented. Needs to be tested

Conflicts:
	docs/sign-low-level-api.md
	keyring.cc
Resolved by Jonas Finnemann Jensen <jopsen@gmail.com>, who cherry-picked
this commit to ensure proper attribution.
Ported cherry-picked detached signature support to node 0.11 with nan.

Also added public API for using detached signatures.