[#18] fixed group/org objects
Now attempting to add meaningful errors to low-level client error messages
Improvements to groups/orgs management in high level client

- Now handling stupid format for organizations extras:
  {'mykey': 'myvalue'} <-> [{'key': 'mykey'}, {'value': 'myvalue'}]

- New methods to get objects by id or name

- list_{organizations,groups} -> list_{organization,groups}_by_name
  as the API return the *names*, **not** ids..

- Hack to list object/organization IDs instead of names
  (requires iterating all of them!!)
[#16,#17] New tests for group/org crud
[#14] Merged dataset CRUD tests in a single module

This is to prevent useless recreation of database over and over
while running tests..