Implement database

[dragfyre]

We should have a database to track the following things:

• user presence for /seen and /users commands
• custom emoji overrides per server

[dragfyre]

After some consultation, it seems that implementing a database is not really that relevant for the time being. The main reason we even considered it was because there were problems with detecting user presence and activity for /seen and /users (see #4), but this seems to be working better in the current version of Discord.js, making it moot. It would be a good idea eventually though, especially if we want to make it possible to support many servers.

Basically, this issue is more worth keeping in mind for future. As such, I'm removing it from Release 1.1; we can leave this issue open for discussion and future development.