Fix Shipping Method Calculator form prototype handling

Currently, switching the calculator field in the ShippingMethodType embeds calculator config prototypes under __name__ key, which causes the form to not validate. This commit makes the following changes:
 * use the proper field name (configuration)
 * skip embedding forms for calculators that don't support configuration (currently results in empty text field being embedded)
fixing spec
making CS consistent
Merge pull request #347 from Richtermeister/fix-shipment-calculator-config

Fix Shipping Method Calculator form prototype handling