core: bridge can now be changed in conf

by default DBus bridge is used, but this can be changed in sat.conf's DEFAULT section using "bridge" option.
    bridge can be "dbus" (or equivalent "dbus_bridge") or embedded at the moment.
    Bridge name used is saved in SAT.bridge_name, and load_bridge method can be used by frontends.

src/tools/common/dynamic_import.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python2
+# -*- coding: utf-8 -*-
+
+# SàT: a XMPP
+# Copyright (C) 2009-2016 Jérôme Poisson (goffi@goffi.org)
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+""" tools dynamic import """
+
+from importlib import import_module
+
+
+def bridge(name, module_path='sat.bridge'):
+    """Import bridge module
+
+    @param module_path(str): path of the module to import
+    @param name(str): name of the bridge to import (e.g.: dbus)
+    @return (module, None): imported module or None if nothing is found
+    """
+    try:
+        bridge_module = import_module(module_path + '.' + name)
+    except ImportError:
+        try:
+            bridge_module = import_module(module_path + '.' + name + '_bridge')
+        except ImportError:
+            bridge_module = None
+    return bridge_module