Added ability to cache the results of database queries.

This is a manual thing. It doesn't happen automatically.

(It's also currently untested)
Removed non-MySQL stuff from the DB class.

I've made the executive decision that I'm only going to support MySQL
for the time being, as that's the only database I'll be using with this
framework.
Changed my mind on making DB MySQL-specific.

I'm not going to write the non-MySQL code myself, but I've decided to go
with an "adapter" model for some database code. There's no strict rules
for what adapters should contain though.

The initial use for the MySQL adapter will be a wrapper for the
"describe table" command.