make khal work with different encodings

you can now set the encoding your terminal uses in khal.conf and all
output should be encoded with that encoding. There is also a new option
`unicode_symbols` (by default set to True), which, if disabled, makes
khal not use any fancy unicode symbols.

ikhal still doesn't respect any of these new settings (but keeps using
utf-8 for now). Alse khal --new might no work yet properly with
decodings other than utf-8.
ikhal: started to work on a calendar chooser

this should enable moving an event between different calendars
no need to try this out, it's not working yet