remove dependency on buggy easywebdav

[francoislaurent]

Python 3 support is compromised by issue amnong/easywebdav#28 .
The easywebdav layer could be removed with direct access to the requests library.

[francoislaurent]

The download and upload methods have been reimplemented, fixing the issue.

For now the dependency is maintained.

[francoislaurent]

Suggestion rejected.