Remove an app name to avoid migration error.

Running a migration against plantshare caused a NoMigrations error for
me involving emailconfirmation, which is something we are using
within PlantShare. Like auth, it probably does not need to be
under South's control, and from what I can tell, we never set
it to be there. Removing it from INSTALLED_APPS should let South
skip it when running migrations, avoiding the error. (#527)
Ask for more specific sighting location: an address.

Now a sighting location can either be an address (street, city, state),
or latitude/longitude coordinates. Postal code is dropped, since
it is essentially the same as the former too-general city/state. (#527)