Add method to download a full size image

We still want to be able to retrieve a full size version of a file.

Add method get_full_size_file which downloads the full size version of a given file.
(it makes use of the method to build the full size URL, introduced in de99534).
Add relevant unit test with its data file.
Add exception handling for non existing full size image

The method get_full_size_file (introduced in 4e22835) will fail for an
unexisting file name. The error message returned is not the same
than the one for get_thumbnail_of_file.

Add to get_exception_based_on_api_message the handling of such error
to return a FileDoesNotExistException.

Add unit test testing this implicitly new behaviour for get_full_size_file.

Using [[Special:FilePath]] might not really be a good idea here though.
Future refactoring should really make use of the API.
Add unit tests for failing download_file

The method download_file may fail for the same reasons than the
underlying method get_thumbnail_of_file − unexisting file or
requested width is bigger than source.

Add two unit tests for this this failing behaviour.
Add docstrings & fix style in test module

Add docstrings to two undocumented methods.
Fix PEP8 & PEP257 style.