Clearify StoreController implementation.

Use bridge pattern to refactor StoreController.
The implementation for different platforms (ios/andorid/editor) can be
seperate clearly.
Update README.md
Clearify StoreInventory implementation.
Clearify StoreInfo implementation.
Fix #if directives for instantiation.
Merge pull request #220 from HolymarsHsieh/BridgePattern

Bridge pattern for platform implementation.
Thanks @HolymarsHsieh
documentation work-in-progress
Merge branch 'master' into unity-docs

* master:
  Fix #if directives for instantiation.
  Clearify StoreInfo implementation.
  Clearify StoreInventory implementation.
  Clearify StoreController implementation.
(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.
- Added 'transactionIdentifier' as purchaseToken on iOS
  fixes #216
- fetched latest changes on submodules
Fixed an issue with IABActivity in android-store
Added documentation.
A few small doc fixes.
Added missing parentheses breaking Android build in StoreController.
Added documentation.md and added some inline docs.
fixed mistake in documentation.md
Changed license in documentation.md
Merge pull request #232 from qtbon/master

Added missing parentheses breaking Android build in StoreController.
Merge branch 'master' into unity-docs

* master:
  Added missing parentheses breaking Android build in StoreController.
  Fixed an issue with IABActivity in android-store
  - Added 'transactionIdentifier' as purchaseToken on iOS   fixes #216 - fetched latest changes on submodules
  (ios-store) Fixed an issue with recieptUrl not being set on iOS<7