Compatibility with .NET CLI version of MsBuild

[djanosik]

I've made some changes to make WebCompiler task compatible with .NET CLI version of MsBuild:

• WebCompiler project now targets net46, netstandard2.0 and netcoreapp2.0.
• The BuildWebCompiler.targets now supports .NET Core MsBuild.
• I have also removed the BuildDependsOn attribute, because I think it's better to use the BeforeTargets attribute. The WebCompile target will now always run before BundleMinify and BeforeCompile targets. But it will run only once of course.
• AjaxMin has been replaced with NUglify. It required minimum changes.
• The BuildWebCompiler.nuspec file has been removed.

It should fix issues #328, #237 and #321.

[djanosik]

@madskristensen I am not sure whether I should update the build script. I have no idea how do you handle versioning, etc.

[djanosik]

Ok, I think I've managed to fix the build configuration.

[304NotModified]

@madskristensen can this be merged?

[madskristensen]

Thanks