Schema changes

[benmerckx]

As I proposed in #43. This has some rough edges to it, but I wanted to check if this is the right approach. I cleaned up the node and php drivers a bit too, because they were becoming unwieldy :) Two things I noticed while going through the code:

• We should support regular indexes (not just primary/unique)
• Maybe Format should really be an instance so it can be subclassed for other database engines?

[benmerckx]

I believe this is mergeable now :)

[back2dos]

I guess a few more tests wouldn't hurt, but we gotta start somewhere :)

[benmerckx]

Noted. I think the tests could use a few tweaks to more easily work on them, let me start there :)