Rename, hide configuration fields behind interface.

Change "Group" to "Collection", and make it an interface instead of a
struct to avoid exposing the underlying implementation details.

To support configurability, add With* methods for adjusting Collection
settings.

Fix go-pg#37: Add Group for configurable groups.

Support registering and running multiple groups of migrations in the
same process, with configurable registration behavior and migration
table name per group.

Fix go-pg#37: Add Group for configurable groups.

Support registering and running multiple groups of migrations in the
same process, with configurable registration behavior and migration
table name per group.

Handle an error

Handle an error

Fix check for new Go version

Add changelog

migrations.DB = orm.DB

Remove init command

Merge pull request go-pg#19 from zeroviscosity/master

Providing non-zero default for newVersion