Add a way to disable automatic schema creation (#106)

* Don't try to create a schema that exists

This commit removes the behavior of executing `CREATE SCHEMA IF NOT EXISTS`
when the schema already exists. This would fail when the schema is created
beforehand (by a superuser) and the current user doesn't have the `CREATE`
privilege on the current database, hence is not allowed to create the schema.

We do keep the `IF NOT EXISTS` part of the query to avoid a (probably very
rare) race condition in which the schema wouldn't exist during the initial
check but would when attempting to create it.

* Remove special handling of the "public" schema

We now do attempt to create it if not exists.

Note that the `CREATE SCHEMA IF NOT EXISTS <schema>` query is now
always run at init, even if the schema is "public".

Move filepath.Abs

Update dependencie

Update dependencie

Merge pull request #89 from TankerHQ/max/allow-select-version-during-…

…migration

Allow collection.Version() calls during a migration

Update go-pg

v8

Update deps

readme: update

Merge pull request #83 from vovainside/master

Update compatibility with CockroachDB