Test for DbSync
refactoring: dbtool now only operates on one calendar

also started renaming `account` -> `calendar` (account was a leftover
from pycarddav)
refactoring
more refactoring
unused code elimination
get_vevent_from_db() -> get()
typo
bugfix
events deleted from the vdir should now be deleted from db as well

this should fix #106 on github
khalendar.Calendar docstring
unused code elimination
more refactoring in backend