Newer versions of Django Haystack changed the location of the variable

for retrieving the current signal processor. While the search code

itself is not sensitive to this, the unit tests are.
This change adds a function for retrieving the signal processor,

grabbing it from Haystack's AppConfig (if present) or using the old

location on the haystack module.

Unit tests pass on Django 1.6 and 1.11 (with other in-progress changes).