Issue #7 pull request

In response to issue #7. Updates wikitools/wiki.py to exit python instance with `sys.exit()` rather than just returning `False` when it fails to log in.
Two problems here.

First is that `sys.exit` quits the program, so returning its value is nonsensical.
The second one is more fundamental: This is wikitools/wiki.py, so it is part of the wikitools library. A library is not a program, and therefore it should never perform something that a program should be doing. Quitting is something that a program should be doing, or at least should have full control over; a library should never make that choice for the program. Thus, you must change the code such that this library code notifies the program code about the failure condition, rather than taking the action that it assumes the program should be taking because of this failure condition.