Separate cmd package for easier importing

[amacneil]

Refactors dbmate into a package which can be imported, and a separate command which uses that package. Based on #10, main changes are:

• Kept package files in root directory so that it can be imported as github.com/amacneil/dbmate (also means you can use the commands as dbmate.Foo instead of pkg.Foo)
• Refactored package & tests so that they have no dependency on urfave/cli
• Added dbmate.DB object which can be used to call high level functions

Example of using this package:

u, _ := url.Parse("postgres://foo")
db := dbmate.NewDB(u)
db.Migrate()