Several improvements

Added several improvements, with test and documentation to the gem.
Including:
+ Readable:
  - when readables have read_mark_id, it will first verify that the read_mark_id is from the same user
  - Added read_by scope
+ Reader:
  - created have_not_read? instance method
  - created with_read_marks, have_read and have_not_read scopes for the user