Continued from PR 1254 (#1296)

* Changes Umbraco.Core to use project.json for package references, instead of nuget (next is the Web proj)

Changes the build to use correct version of MSBuild
Updates the Web.UI project to reference Microsoft.CodeDom.Providers.DotNetCompilerPlatform so that we can use c# 6 in views
Updates all other projects to use auto-package restore instead of the legacy nuget target restore

* Updates Umbraco.Web to use project.json - removes the lock file (will ignore)

* updates ignore to ignore lock files

* forgot about the tests project, removed the old nuget targets

* Need to restore some packages before we can build

* Update to latest nuget.exe which understands project.json

* This happened automatically, I'm guessing new NuGet removed these

* Correctly restore all necessary packages

* Fix merge problems and package version dependency conflicts

* Removes unnecessary file

* Removes unnecessary file

* Some things didn't make it during the merge, re-applying

* And even more changes that got reverted

* And even more changes that got reverted

.gitignore

@@ -136,6 +136,8 @@ src/Umbraco.Web.UI/js/*
 src/Umbraco.Tests/media
 tools/docfx/*
 apidocs/_site/*
+src/*/project.lock.json
+
 apidocs/api/*
 build/docs.zip
 build/ui-docs.zip