This is necessary because Gajim is with 0.16.11 a python package

- Prepare the Plugin so it works with Gajim as a package.
- Add a Fallback method so we can update Plugins that are not loaded
by the PluginManager

This reverts commit 346da609

current Gajim version. Fixes #225

- Print only warning instead of error, as the module is only optional
- Log the error to the debug log, so users dont think we didnt catch the exception

The HTTPUploadPlugin calls that if encryption is enabled

Call the `encrypt_file` method of the encryption plugin instead

A patch in Gajim made that unnecessary

- unused import
- unused var GAJIM_VERSION
- redundant self.plugin = self
- simplify condition
- redundant 'else'

This happend during porting the plugin to the new encryption API