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.
Remove duplicated --schema option for tenant_command
doc update
Allow to store template in a subdirectory of a tenant directory

For example use MULTITENANT_TEMPLATE_DIRS =
('/var/lib/tenants/%s/templates/',) instead of just
('/var/lib/tenants/templates/',).
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...
PEP8 and typos
Merge pull request #172 from rev112/pep8_and_typos

PEP8 and typos
Merge pull request #165 from rev112/fix_duplicated_tenant_schema

Remove duplicated --schema option for tenant_command
Added third party apps section to dos
Merge pull request #167 from bdauvergne/wip/improve-template-loader-path

Allow to store templates in a subdirectory of a tenant directory
Add validator for TenantMixin.schema_name, check for 'pg_' prefix
Convert README to reStructuredText (.rst)
Merge pull request #176 from rev112/readme_to_rst

Converted README to reStructuredText (.rst)