Golint compatible migrations

[d]

Goose's automatically generated migration functions Up_xxxx() and Down_xxxx() are exported, but they don't have godoc comments, and thus golint complains about them. It would be preferable to un-export them (if possible), or at least add a boilerplate comment to shut golint up.

Golint also complains about underscores in function names; it would be nice to get rid of those as well.