Add Core.SharePointProxyForSpaApps

I'm not sure if I created the pull request properly, kind of new to GitHub.  This is a sample I created which demonstrates an AngularJS Single page application making requests to App web or Host web. The requests are proxied through a service from app's server to append access tokens which preserves security around access tokens.  Normally developers would compromize the security by storing the token in a cookie so the client could have access which is not a very good practice. Also, people don't have to worry about using the dreaded RequestExecutor. Seems like a good end-to-end solution that other developers could use as a foundation for their applications. Read the included word docx for more details

Original repo is here: https://github.com/mattmazzola/SharepointProxyForSpaApps