Take absolute path of self.output_path.
Take absolute path of self.output_path.

This fixes bug in issue #298 where the command 'lektor server -O <relative_path'
can cause an infinite pruning/build loop. Previously, it was comparing
the absolute watch path to the relative server output_path, causing the
is_interesting to return 'True' when it shouldn't.

lektor/lektor,lektor/lektor,lektor/lektor,lektor/lektor