relative imports
make sure month names are unicode

calendar.month_abbr returns str (bytes) in python2 and str (text) in
python3, if a locale with day or month names containing non ascii
characters was used this lead to a UnicodeDecodeError.

This should fix #142 on github.
more python3 compat

khal agenda/calendar/new and even ikhal work under python3, parsing
vdirs still leads to errors though (see tests)