Unify action API
Factor out prefetching
Fix tangled logic
Import beautification
Port CLI to Click.

After a while i finally gave in and ported the whole command line
interface of vdirsyncer to Click. While it might seem like a huge
dependency, i hope we can eliminate some helper functions in the
process, and maybe are more motivated to write a more beautiful and
intuitive interface due to all the niceties Click provides.
Add test for simple usage
More beautiful output
Only add stdout handler when running as cli
Add one more test
Catch exceptions with decorator

This allows us to validate the exact output in the tests
Don't use click's newline echoing

click.echo does two write calls, which get flushed immediately. Using
this in multiprocessing might make output overlap. Using a single
write-call apparently resolves that issue, although i don't know why.

A proper solution would be to use locks, but e.g. multiprocessing's
logger doesn't seem have nearly as much flexibility as others. For now
it seems to be bugfree enough.
More tests