Add support for datetime columns
Base64 encode/decode cursor to hide it's content
Make separate Cursor object to get out messing

Improve structure, convert and encode any cursor to YAML/Base64
Add Feature test for cursor pagination
Support multi columns cursor pagination

New API is further
    Entity.cursor(params[:cursor], cursors: { id: { reverse: false },
    created_at : { reverse: false } }).per(20)
Updated README to reflect last changes