- Add a option that allows to disable HTTPS verification

We would receive a KeyError for jids we dont have in the roster,
now we skip them and print a warning.

Fixes #173

[httpupload] Use 'max-file-size' information

See merge request !44

If a file is too large, use the 'max-file-size' value to generate an
error message rather than using the error message provided by the XMPP
server. This enables easy i18n of the error message and a
human-readable file size.

This is due a update to ejabbered where Nodes can now hold 10
items per default

Use absolute imports on all plugins

See merge request !43

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