Use new JAuthenticate constants for Joomla! 3.0

In 3.0 the JAUTHENTICATE_STATUS_* constants were removed and new
constants were added to JAuthenticate.  These are now the ones that we
need to use.

Signed-off-by: Kevin Locke <klocke@digitalenginesoftware.com>
Use Joomla! translations for auth messages

Add the standard "Authentication failed:" prefix to the authentication
errors, as is done in many of the other authentication plugins.  Also
replace our "password must not be empty" message with a standard Joomla!
one.

Signed-off-by: Kevin Locke <klocke@digitalenginesoftware.com>
Update author information

Since Digital Engine Software hasn't existed for a few years, there's no
sense listing it as the author/maintainer.  Update it to use my personal
information since I'm continuing to update it.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Configure GitHub as an Joomla! Update Server

To allow users to get automatic updates, add the appropriate XML files
to the repository and point the updates to the version of the file on
the master branch.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>