[RFC] database refactorization 

Roughly work in progress, actually I want to start working on GitLab HQ, or mercurial (yay) integration but realize there's a lot of yak shaving need to be done first.

This includes refactoring database setup, and migration system.

Remove setupDatabase from main and use `Init` method from database
package.

At database package, defines Init method which actually initiate
database with options given from comand line flag. I think `--path`
wont be used anywhere so I plan to remove it later.

Both meddler and migration initiated here (`database.Init`), then we call `Set` method
to setup all the tables, etc. Here I think I want to separate database
schema and turn it into migration script instead, later maybe.

At migration package I made some tweak to `Operation` interface.
Realized that it's ludicrous to let migration driver re-implement `Exec`
and `Query`, I made migration script to receive The whole
migrationDriver struct which contains both Operation implementer, and
the Tx itself. This made possible thanks to Go struct being able to
promote its member, now our migration is more transparent.

There's also stub implementation for both mysql and postgresql, will
implement this really soon.

I also want to discuss the idea of separating migration system out of drone binary, does it makes sense if we make drone oblivious about migration, and use separate binary to do the migration instead?

Thanks!