Added .cache attribute on throttles.

Closes #1066.  More localised than a new settings key, and more
flexible in that different throttles can use different behavior.
Thanks to @chicheng for the report! :)

docs/api-guide/throttling.md

@@ -70,6 +70,13 @@ Or, if you're using the `@api_view` decorator with function based views.
 
 The throttle classes provided by REST framework use Django's cache backend.  You should make sure that you've set appropriate [cache settings][cache-setting].  The default value of `LocMemCache` backend should be okay for simple setups.  See Django's [cache documentation][cache-docs] for more details.
 
+If you need to use a cache other than `'default'`, you can do so by creating a custom throttle class and setting the `cache` attribute.  For example:
+
+    class CustomAnonRateThrottle(AnonRateThrottle):
+        cache = get_cache('alternate') 
+
+You'll need to rememeber to also set your custom throttle class in the `'DEFAULT_THROTTLE_CLASSES'` settings key, or using the `throttle_classes` view attribute.
+
 ---
 
 # API Reference

rest_framework/throttling.py

@@ -2,7 +2,7 @@
 Provides various throttling policies.
 """
 from __future__ import unicode_literals
-from django.core.cache import cache
+from django.core.cache import cache as default_cache
 from django.core.exceptions import ImproperlyConfigured
 from rest_framework.settings import api_settings
 import time
@@ -39,6 +39,7 @@ class SimpleRateThrottle(BaseThrottle):
     Previous request information used for throttling is stored in the cache.
     """
 
+    cache = default_cache
     timer = time.time
     cache_format = 'throtte_%(scope)s_%(ident)s'
     scope = None
@@ -99,7 +100,7 @@ class SimpleRateThrottle(BaseThrottle):
         if self.key is None:
             return True
 
-        self.history = cache.get(self.key, [])
+        self.history = self.cache.get(self.key, [])
         self.now = self.timer()
 
         # Drop any requests from the history which have now passed the
@@ -116,7 +117,7 @@ class SimpleRateThrottle(BaseThrottle):
         into the cache.
         """
         self.history.insert(0, self.now)
-        cache.set(self.key, self.history, self.duration)
+        self.cache.set(self.key, self.history, self.duration)
         return True
 
     def throttle_failure(self):