Fix load error if active_support/core_ext was not loaded before.

ActiveUtils requires some ActiveSupport core extensions in its main file.

Error:
active_utils-2.2.3/lib/active_utils/common/post_data.rb:5:in `<class:PostData>': undefined method `class_attribute' for ActiveMerchant::PostData:Class (NoMethodError)

Closes #99
reapplying realex gateway addition changes
switch to service_url
Fix to realex timestamp generation
Amend changes to service_url to enable tests to pass
Whitespace fix and remove unnecessary line comment
Test Realex format_amount_as_float
Remove previously commented assertion from Realex notification test
Remove reference to Shopify
Use mocha stubs instead of override of OffsitePayments.mode
Implement billing/shipping code preparation for Realex AVS check