pithos: Remove obsolete billing code
pithos: Remove obsolete code

The backend methods _report_object_change and _report_sharing_change
had been introduced in order to set asynchronously the object's MD5 hash
and for notifying users that somebody has shared an object with them
respectively,
These methods used to send messages to specific queues (if such queues
existed).
These procedures although implemented, they have never been documented
or used and since commit 8b3df8e60618611cd2026d03afcd88f7e5708057
they have become useless.
This commit eliminates them as well as the 'dispatch' module
(used for consuming such messages).
Also, it removes 'details' argument from _report_size_change method
since it was used mainly for passing additional information
to the billing system
(actually it has been substituted by the 'name' argument).
pithos: PEP8 fixes
pithos: Backend refactoring

Remove 'pithos.backends.base' module:
* Move backend method docstrings to ModularBackend class.
* Define backend exceptions in a separate module.
pithos: Provide descriptions for backend errors

Closes #1
pithos: Handle backend errors centrally

Since commit 122527366668825f4c9e63de9f3a2236d94006f9
the backend provides meaningful descriptions for backend exceptions.
This facilitates the frontend to handle centrally such exceptions.
pithos: Cleanup 'pithos.api.util'

Improve code's readability
pithos: PEP8 fixes