Sign artifacts from Gradle build

This change successfully configures Gradle to sign artifacts with the
signing plugin and its `signArtifacts` task, per documentation at
http://www.gradle.org/docs/current/userguide/signing_plugin.html.

However, it is currently not clear how to actually upload the resulting
signatures when using the newer 'maven-publish' plugin (as opposed to
the older 'maven' plugin).

The thread at http://stackoverflow.com/a/16617976/622403 discusses the
problem. Note my follow-up question to Peter's original answer.

In the meantime until there is a proper solution, we don't strictly need
artifacts to be signed: not until publishing the first release through
Bintray to the Central Repository.