Allow easy database configuration

I was in need to adapt the DB structure, and I came up with this update to allow developers to customize it without altering core files, and still provide an out-of-the box schema for quick deployment.
Example of changes made in my case:
- Renaming of the tables to prefix them by "oauth_" (database already contains a lot of tables)
- Columns addition in the clients table, to store name, contact info, etc...

PS: I don't know all Github subtilities, it seems it has included in this pull request all the previous commits in my master branch (those of my previous pull request). The feature I'm talking about is in the last commit (8907078).