Add Tasks and Build-Script from https://github.com/matthid/Paket.TeamBuildCredentials

[matthid]

This PR:

• Adds a Build-script to build the credential-manager and the final vsix
• Adds the tasks from https://github.com/matthid/Paket.TeamBuildCredentials

Some further changes I'd like to do (possibly in future PRs):

• Add a FAKE 5 task (with automatic setup and cleanup of credential-manager built in)
• Add credential-manager support for the "Paket" task (with automatic setup and cleanup of credential-manager build in)
• Change the "Paket" Task to not depend on a checked-in bootstrapper

[isaacabraham]

Awesome. I'll review this in next day or so and then merge in :-) Thanks so much!