[plugin_installer] Rewrite plugin

- Use libsoup for HTTP operations
- Add new config dialog
- Move code into smaller modules

plugin_installer/remote.py

@@ -0,0 +1,8 @@
+# File which defines all remote URLs
+
+server = 'https://ftp.gajim.org'
+directory = 'plugins_master_zip'
+
+PLUGINS_DIR_URL = '%s/%s' % (server, directory)
+MANIFEST_URL = '%s/manifests.zip' % PLUGINS_DIR_URL
+MANIFEST_IMAGE_URL = '%s/manifests_images.zip' % PLUGINS_DIR_URL