Add a class cache to avoid runtime extend calls

* Strainer has a class cache that creates Strainer subclasses for each filter
   set that is used on .create calls.
 * Context now creates a list of filters and passes this to Strainer.create to
   utilize the class cache in almost all use cases.
 * If add_filter was called after a render, then the method cache may still be
   invalidated.

Conflicts:

	lib/liquid/strainer.rb

test/liquid/strainer_test.rb

@@ -49,4 +49,15 @@ class StrainerTest < Test::Unit::TestCase
     assert_equal "has_method?", strainer.invoke("invoke", "has_method?", "invoke")
   end
 
+  def test_strainer_uses_a_class_cache_to_avoid_method_cache_invalidation
+    a, b = Module.new, Module.new
+    strainer = Strainer.create(nil, [a,b])
+    assert_kind_of Strainer, strainer
+    assert_kind_of a, strainer
+    assert_kind_of b, strainer
+    Strainer.class_variable_get(:@@filters).values.each do |m|
+      assert_kind_of m, strainer
+    end
+  end
+
 end # StrainerTest