api keys: allow users to have multiple API keys.

This is useful if you have multiple programs and want to give them
different API keys, or if you want to rotate keys for a single program.

test/unit/api_key_test.rb

@@ -25,7 +25,7 @@ class ApiKeyTest < ActiveSupport::TestCase
 
     should "have the same limits whether or not they have an api key" do
       assert_no_difference(["@user.reload.api_regen_multiplier", "@user.reload.api_burst_limit"]) do
-        @user.api_key.destroy
+        @api_key.destroy
       end
     end
   end