Add Ecto.Repo.one which returns either zero or one result.

If there are two or more results one raises Ecto.NotSingleResult.

Also Add Ecto.Repo.one!, change Ecto.Repo.get to have no limit and add
Ecto.Repo.get!
Merge pull request #210 from MSch/repo-one

Add Ecto.Repo.one which returns either zero or one result.