Product sources build against MyGet delivered framework contracts.

Previously, the product sources have been compiling against the
reference assemblies provided by the environment.  These were the
portable targeting pack reference assemblies.

This change adds package dependencies from each of our product
assemblies to .NET Core contract assemblies on a new MyGet feed at
https://www.myget.org/F/dotnet-corefx.  This change also modifies the
assembly projects to not implicitly reference the reference assemblies
using the ImplicitlyExpandTargetFramework property. Finally, this
change explicitly references framework contract assemblies in their
location relative to the package root for downloaded nuget
dependencies.
Revert change in commit 03e79fe to Immutable and MetadataReader

Revert the change "Product sources build against MyGet delivered
framework contracts" from System.Collections.Immutable and
System.Reflection.Metadata while keeping the changes for the rest
of the assemblies.  Immutable and MetadataReader are previously
released targeting portable targeting packs, and this change would
narrow which platforms they would run on.
Update framework references for product, add local refs for tests

Update the framework references for product assemblies to the correct
(updated) folder structure of the framework *.nupkg files (with a +
separating platforms rather than a -), and also add references to
nuget delivered .NET Core framework contracts to test assemblies.
Fix SIMD test and disable XML tests on issue corefx/208

https://github.com/dotnet/corefx/issues/208 tracks running tests
against the implementation assemblies for .NET Core instead of the
desktop assemblies.  Disable some tests from running because they
compile against a new overload in the System.Runtime contract
(System.String System.String.Format(System.IFormatProvider,
System.String, System.Object) that does not exist in the desktop
implementation assembly (which only has a params overload).
Merge branch 'packageref'

Conflicts:
	src/System.Xml.XPath.XDocument/tests/System.Xml.XPath.XDocument.Tests.csproj
	src/System.Xml.XPath.XmlDocument/tests/System.Xml.XPath.XmlDocument.Tests.csproj
	src/System.Xml.XPath/tests/System.Xml.XPath.Tests.csproj