implemented simple shared secret authentication using the http
authentication header.

- Added flag for -secret. If -secret is an empty string (default), the
  skydns server does not require authorization.
updated readme for shared secret
quick doc update
Added auth for external api
Added tests for success and auth failure.
Merge branch 'master' of https://github.com/skynetservices/skydns

Conflicts:
	server_test.go
added myself.