(ios-store) Fixed an issue with recieptUrl not being set on iOS<7

fixes #217
Merge branch 'master' of github.com:soomla/unity3d-store

* 'master' of github.com:soomla/unity3d-store:
  Fix #if directives for instantiation.
  Clearify StoreInfo implementation.
  Clearify StoreInventory implementation.
  Update README.md
  Clearify StoreController implementation.