Allow setting the configuration directory on command line

This allows a workaround for issue https://github.com/owncloud/mirall/issues/166

Running multiple instances of the owncloud client with different configuration directories allows to connect to multiple servers without the client having to directly support it.
So it's a workaround and a quick solution for those like me that need to connect to multiple servers.

I implemented it and ran the client but unfortunatelly I couldn't test the functionality, because the git HEAD version aborts wth a segfault on my computer (also without my changes).
So I ask you to test it before accepting the pull request. I think it should work for I didn't change much.