Fix external links in Scaladocs.

This addresses issue #41, by just taking the path of least
resistance and just entering explicit URLs for the three
external references we currently have.
Updated external documentation links.

* Link specifically to Akka 2.2.1 apidocs for Agent.
* Point to Akka 2.2.1 configuration docs, which are more useful than
  the (nonexistent in 2.2.1) NettySSLSupport class's apidocs. :)
Merge pull request #82 from comcast-jonm/scaladoc-warnings

Fix external links in Scaladocs.