Updating core sdk to V1.2.0 & fixing XML comments for better intellisense support
Updating web.config to use mode based API endpoint configuration
README cleanup
  - Moving NuGet installation instructions to wiki
  - Adding SDK configuration instructions
Fixing typo in README.md
Adding custom configuration support for IPN messages
updated samples for 100 release, error language became non mandatory param in request envelope
updated stubs, Changelog.txt and version for 100 release