Ensure collectors variable is initialised

If no instances are detected by pg_view.py then an UnboundLocalError occurs in the finally clause of main(). To fix this initialise the `collectors` variable outside the try block, guaranteeing it's initialised before the finally: block can execute.
Merge pull request #8 from moreati/patch-1

Ensure collectors and group variables are initialized. This fixes the Exception when pg_view is started on the host with no detected (or running) databases.