Fix for occasional crash due to unsupported URI schemes

[AndrewGretton]

We have recently begun using Premailer in a high traffic environment (and we love it). We've run into a handful of users who have inadvertently injected client-side CSS links, and currently Premailer throws an exception when it tries to inline it. That's because underneath, it uses WebClient, which only supports four URI schemes.

Here's a suggested fix which only attempts to download from the four supported schemes, including a supporting test. Would love any feedback! Thanks.

[martinnormark]

Thanks for this 🙇

[AndrewGretton]

@martinnormark any chance of pushing this (and any other fixes) to Nuget? Would love to get them included. Thanks!

[martinnormark]

@AndrewGretton New release is cooking 🍤

Build: https://ci.appveyor.com/project/milkshakesoftware/premailer-net/build/164

Should push to Nu Get in a few minutes.