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.
This is too confusing, we're getting two defects with different levels for one error ([example](http://i.imgur.com/WqIF8xo.png)), better solution would be to append the explanation to the preceding error. You can insert `\n` between error and an explanation, Visual F# spits out multiline errors and they seem to work just fine.