Speed enhancement for set search_path()

* Refactor out of _cursor() to perform `set search_path()` once during a request cycle.
Revert to setting search_path in _cursor()

Ensure called only once, via an instance variable to track.
Make "fast and loose" behavior opt-in

* Add support for a `settings.TENANT_SCHEMAS_PARANOID` variable;
  if false, limit calls to `set search_path`
doc typo
tweak name for consistency with other settings
Rename TENANT_PARANOID

* Rename to TENANT_LIMIT_SET_CALLS and get_limit_set_calls(), per
  pull request feedback.
doc update
Merge pull request #164 from bimsapi/master

Added optional optimization setting TENANT_LIMIT_SET_CALLS.
Does not try to import south syncdb command if south is not in INSTALLED_APPS

If south is not really installed it break classic configuration since
the SOUTH_ADAPTER is not configured.
Merge pull request #170 from bdauvergne/wip/fix-south-import

Does not try to import south syncdb command if south is not in INSTALLED...