Make a main class OfflineImap that is being called

Rather than calling a function in a module, invoke offlineimap by
calling an OfflineImap object.

This removes code lying outside of objects; I prefer to keep code
within an object and provides us with a nicer Object encapsulation.

It will also ease the testing of Object functionality in unittests
when they are introduced.

Previously we would import and start Offlineimap like this:

from offlineimap import init
init.startup('6.2.0')

now we do:
from offlineimap import OfflineImap

offlineimap = OfflineImap()
offlineimap.startup('6.2.0')

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/__init__.py

@@ -1,3 +1,5 @@
+from offlineimap.init import OfflineImap
+
 __all__ = ['ui', 'folder', 'repository', 'mbnames', 'threadutil', 'init']