Replace ErrorTask with DocumentTask in build logger

Fixes #40

I also extended the Rustc wrapper to parse `note:` messages as warnings instead of errors.

I don't know where the code in Microsoft.VisualStudio.Project comes from, but I had to modify it and add a new reference to the `Microsoft.VisualStudio.Package.LanguageService` assembly, because tho `DocumentTask` class is defined there.
I had a test where the output of rustc was the following: https://gist.github.com/Boddlnagg/5b2ad19722bdced23fa6
The regex as it was before didn't work correctly with line 11/12 (it parsed the filename as  `...\nmain.rs`). I don't understand the exact reason, but this fixed it.