Better defaults for artifact signing

This patch doesn't change anything if the signing.keyId,
signing.secretKeyRingFile and signing.password properties are
configured, but it provides better defaults if they are not configured:

- If signing.keyId is missing, a more helpful error message is shown,
  explaining to the user how they can configure the property.
- If signing.secretKeyRingFile is missing, it defaults to the default
  location of GnuPG's secret keyring file on Unix-like systems.
- If signing.password is missing, it prompts the user. This avoids
  having to store your GPG passphrase in an unencrypted file on disk.

This is my first time attempting any custom Gradle build configuration,
so my approach may be way off -- if so, I'd love to see how to do this
better :)

Also, sorry for lack of tests. gradle-nexus-plugin's integration tests
fail for me (even before making any changes), so I wasn't sure where to
start.
Bumped up version and changed release notes.