remove links to missing locations

Since we are do not have data files for some locations, "links" to
them (signposts and doors in adjacent locs) need to be removed to
prevent all sorts of errors.

* add pers.exists function for *very specific* use cases like this one
* return null on pers.load/get for nonexistend objects, instead of
  treating this as an ERROR
* log attempts to load nonexistent objects at INFO level (because in this
  scenario it is expected behavior, but we still want to know if this
  occurs in other situations)
* remove connects to nonexistent locations while loading/initializing a
  Geo instance
* adjust tests accordingly
Merge pull request #23 from yelworc/missing-locs

remove links to missing locations