NuGet v3

[brywang-msft]

(Opening this so Jianyun can leave some comments before vacation. Additional changes may be pushed after closing this.)

• NuGet v2 was refactored to allow for v2/v3 selection
• NuGetPackageRepository introduced as the base package repository for NuGet operations
• NuGetResourceCollection introduced as the collection of v2 or v3 code
• Most of the low level code is refactored then reused, like downloading package
• RequestWrapper introduced that allows for code to run without NuGetRequest object