Wimsapi

* Default timeout for low level API is now 10 seconds (instead of 120).

* Parameters of `api.py` requests are now encoding in `ISO-8859-1`, mathching WIMS'
default encoding

* Adding `__repr__` and `__str__` method to `Class` and `Item` subtypes.

* Getting Exams from the WIMS server now retrieve the correct status.

* Fix sheet's score computation

* Added classes `Exam` and `ExamScore`, `ExerciseScore` and `SheetScore`
to store scores

* `Sheet` / `Exam`:
* Title and description are now optionnal in constructor.
* Added method `scores(user=None)` to retrieve the score of one or every user.

* Added class method `check()` to `Class` to check wheter a class exists or not.

* Better `__eq__` and `__hash__` for every class.

* Listing functions now return an empty list when needed

* Added new item : `Sheet`
* Added the possibility to list items and classes through `Class.list()` and
`class.listitem()`.
* Added `__eq__()` for items and classes.
* Fixed some documentation