add template loaders looking for template based on the current tenant

CachedLoader is similar to the django.template.loaders.cached.Loader but
it adds the tenant instance id to the key used for caching.

FilesystemLoader is similar to django.template.loaders.filesyste.Loader
but it uses the setting key MULTITENANT_TEMPLATE_DIRS instead of
TEMPLATE_DIRS and looks for template inside the directory
os.path.join(MULTITENANT_TEMPLATE_DIRS, tenant.domain_url).
Merge pull request #103 from entrouvert/master

added template loaders for finding templates based on the current tenant