Support :binary_ids via cursor paging

[danielberkompas]

mix cloak.migrate now supports the most common forms of :binary_id via cursor paging. (e.g. PostgreSQL UUIDs) The only requirement for the binary ids is that values must be sortable/comparable using >. (PostgreSQL can do this for UUIDs)

If your primary key cannot meet this requirement, you can specify custom cursor fields using Cloak.CustomCursor.

• Each field must be sortable/comparable using >
• The combined value of the fields must be unique for each row

See #71 by @brentjanderson for the initial issue report and solution. Closes #71 Closes #70

[coveralls]

Coverage increased (+0.3%) to 98.101% when pulling 8ecb1596cfbbed1da731871b7ca5f3471ab8cd27 on migrate-with-cursors into 66b98d73e9926e95fd2bb69ab9a9ea3d093346f0 on master.