Add support for RDS tags

With this patch, billow now fetches the tags for RDS instances.  Turns out, this isn't the easiest or cleanest thing to do.

I also have not java-ed in a long time, so I may be java-ing very wrong.

/cc @nelgau @andykram (for java)
/cc @ktrice (because he touched it last)
/cc @pcarrier (because this is open source and :heart:)
Checking null-ity of params probably couldn't hurt.