FTBFS with recent guacamole (master or unstable)

Four changes:

1. Guacamole expects prefixes on connection ids - `"c/"` for a connection and `"g/"` for a connection group. This is now fixed; though it is somewhat hacky as the proper way to handle this requires use of a private `enum`. The fix currently assumes that all prefixes will have a length of 2 characters.
2. The `add` method in `SimpleConnection` is a stub which throws a 'Permission denied' exception - so I could connect through Guacamole one time only, then `updateUserContext` is called and throws the exception. Changed to use `putConnection` instead - which also has the added benefit of allowing connection names to be reused, as it will replace a connection of the same name.
3. The packages built from the git repo (both master and unstable) use the namespace `org.glyptodon.guacamole` and not `net.sourceforge.guacamole` for everything except the auth plugins themselves. This plugin will not build against recent sources without this change. I also changed the pom.xml to use this namespace for dependencies, though, which I'm guessing means it won't pull packages from the SF repository, but does pull from the local Maven repo. But since the SF packages right now won't work anyway, I figure this is better.
4. Not everyone is a Java programmer (I'm not). ;) Added instructions to README.md to make it easier to build for those who are not familiar with Java or Maven.

But it's all working for me now. :) Thanks a lot for this plugin! This is much better than my previous idea, which was exporting all users and machines from our web application and filling the guacamole-auth-mysql plugin's tables with that data.