Responsibilities cleanup.

Folder  path escaping  responsibility  was moved  to  API Facade  class;
folder class no more tries to generate URL / parameters when loading the
document list.
Removed an unused private method.
Renamed an event.

:document_deleted  is  more specific  name  than  :deleted, as  we  have
different entities which can be deleted.
Added "Delete a folder" functionality.
A minor documentation update.
Added gem version badge to the README file.
Code climate badge was added to README.
"Create a new folder" functionality.
Added some error handling code.

Folder  class now  raises an  error when  user attempts  to add  a child
folder if it already contains the folder with the same name.
Error handling.

New   folder  name   is   compared  with   existing   folder  names   in
case-insensitive manner now.
"Move document to a different folder" functionality.
Milestone end, new version 0.0.8