Improve code quality.

Add comments / TODOs.
Move variables to closer scopes. In the case of the executor, apparently
threads have a reference to the executor which keeps it from being
garbage collected.
Use parameter instead of hardcoded string in VelocityToadlet.
Replace "Infocalypse" with "DVCS".

The plugin is intended for DVCS in general, not just Infocalypse.
Replace "public key hash" with "identity ID".

Though the identity ID happens to be the public key hash / routing part
of the key, identity ID is more semantically meaningful.
Support POST; rename updateContext() to doGet().

updateContext() doesn't make purpose as clear as onGet(). It may still
be confusing that after onPost(), onGet() is called.
Update README; dependency version.