Request intercept and proxy improvements

[rbrundritt]

I've been pushing the limits of this library a lot and ran into a few limitations that I was working around. I'm now coming back and

• Added support for a lot more mime/content types. Maintained the same fallback values as before.
• Improved support for MainFile such that it can be a URL with query string parameters. Allows the main file to point to the proxy.
• Added support for custom response headers via the proxy.
• Cleaned up some repetitive code to ensure more consistency between platforms.
• Extended proxy sample to demonstrate how it can be used to access resources on non-CORs enabled endpoints.
• Added an example wraps the HybridWebView in a class library to create a custom control. It uses the proxy to load the main page and an image as embedded resource. Added a page to the main sample that loads this custom control.
• Improve stability of query string parsing
• Add example that leverages the improvements to MainFile loading and a proxy to load web pages and resources from the file system (app data directory).