Fix some more XML docs
Remove unused static _canUseAsync field

This was previously (Environment.OSVersion.Platform ==
PlatformID.Win32NT) but has since been updated to always be true.
This is now dead code that should be removed.
Canonicalize MSBuild ToolsVersion to 12.0

Right now, there's a mix of ToolsVersion 4.0 and 12.0 in the projects.
We should make sure that it's 12.0 everywhere.
Enable resolving projects to resolve references from packages.config.

This change introduces the new build tools package version of 1.0.17.

Currently the ResolveNuGetPackage task doesn't properly set Private on the
resolved references so not all libraries like xunit are getting placed in
the output folder. That causes runtime issues so to avoid that this nuget
resolution is turned off in the XunitTraitsDiscoverers.csproj which will
cause the xunit libraries to end up in our tests folder and can be used by
all the tests.
Convert XUnitTraitsDiscovers project to package resolution
Convert System.Console to package resolution
Switch all projects to resolve references from packages.config.
Merge pull request #297 from justinvp/pipes_deadcode

Remove unused static _canUseAsync field in PipeStream
Merge pull request #252 from alfaproject/regex_xml_docs

Fix some more XML docs
Merge pull request #310 from weshaggard/NugetRefs

Switch all projects to resolve references from package.config
Switch to one Nuget.Config file in the corefx repo.
Merge pull request #305 from akoeplinger/fix-msbuild-toolsversion

Canonicalize MSBuild ToolsVersion to 12.0
Register msbuild.log as an asset on AppVeyor

If we need to debug the build issues it's pretty convenient if we can
directly download the diagnostic log.
Merge pull request #314 from terrajobst/master

Register msbuild.log as an asset on AppVeyor
Minor cleanup for README.md
Merge pull request #319 from stephentoub/readme_cleanup

Minor cleanup for README.md