Wimsapi

* Renamed Class member `date` to `expiration` to match the *ADM/RAW* argument.
* `Class.limit` is now an *int* when retrieving the class from a *WIMS* server.
* Now propagate exception if expiration in Class `__init__` is not `yyyymmdd`.

* `check_exists` is now used properly

* Added `check_exists=True` parameter to item's save method.
If check_exists is True, the api will check if an item with the same ID
exists on the WIMS' server. If it exists, save will instead modify this
item instead of trying to create new one.
`wclass.additem()` will now use `check_exists=False`.
* Fix response check in `wclass.save()`

* Fix missing `self.lang = lang` in **Class**' `__init__`

* Fix `long_description` in setup.py

* `qclass` argument is now optionnal in Class constructor,
allowing WIMS to choose a free `qclass` when saving for
the fist time.