Started to implement schema.columns

No tests at the moment, but seems to work fine so far. Added it to schema, because from my point of view that's where it actually belongs. @tgriesser, do you agree? It's up for discussion whether to include the numeric precision for numbers or not.

Also, as I already noted in my plugin pull-req over at bookshelf, when doing this in mysql, when you are creating tables on the fly, a `FLUSH TABLES` must be sent prior to fetching columns. Questions is, whether we should have the statement in there always or not.