[person/department] new func to add department or get one by name

[flablog]

Problem
There was no way to create a department from Gazu, neither looking for one by name

Solution
Added 2 functions to enable developers to create departments from Gazu and to search a department by name

[flablog]

This is a PR that we can consider a test as for the questions I'm raising in #329
For eample, there is no tests here, as I'm not sure how to make it.
And I put the functions where there is the only related department function, but maybe it needs its own file ? Like departments.py ?

[EvanBldy]

Hi @flablog,
It's a good idea to let the functions in person's file.
I have added the tests for your functions and added a function gazu.person.get_department.

Thanks a lot for your work, have a nice day :)