Fix get_user_calendar for MySQL backend

It now returns a list of str instead of tuples
Add context from mapistore use username and not cn
Manage no MySQL connections in the OpenChange db wrapper
Send FreeBusy Response even when the FreeBusyOptions are not sent

This is the behaviour Outlook 2007 has. It sends back as TimeWindow
from server side time now up to 42 days later.
Sequential selects on multithreaded environment

* Makes multiple requests at once work as expected
Set proper error codes in each case
Fix dummy bug on creating the lock
Missing import for LDB backend

+ Add copyright
+ Be more pep8 compliant
Merge pull request #35 from sixstone-qq/ejhernandez/freebusy-fixes

Fixes on Free/Busy