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
Add reference from RegularExpressions to XunitTraitsDiscoverers

This is required to make the solution self-contained. It also follows
other solutions that already reference XunitTraitsDiscoverers, such as

* System.Linq.Parallel.sln
* System.Numerics.Vectors.sln
* System.Reflection.Metadata.sln
* System.Text.RegularExpressions.sln
* System.Threading.Tasks.Dataflow.sln
* System.Xml.sln
Ensure building in Visual Studio works without having to run build.cmd

Currently, you need to run build.cmd once before you can open and build
any of the solutions in Visual Studio. The reason is that the project
targets have dependencies on NuGet packages that aren't expressed via
a packages.config. So while the Visual Studio NuGet is able to restore
NuGet packages, it can't restore those because it doesn't know about
the build-tools dependency.

I've fixed that by adding a top-level .nuget folder which is recognized
by NuGet as global dependencies. The only reason I've added this folder
to each solution is because the NuGet client will add it automatically
to the solution on first build. So to avoid future noise, I've added
the file to each solution already.

Also, in order to make the solutions build immediately, we can't rely
on the build tools being copied to $(OutDir)tools, which is what the
current build process does. Instead, I've changed the build to directly
run from the package location.
Merge pull request #324 from terrajobst/vs-build

Ensure building in Visual Studio works without having to run build.cmd