Automatically OSS sign binaries during build.

Automatically OSS sign (delay sign and then set the "signed" CLI
header flag) binaries during build.  The "SkipSigning" property can
be set by an individual project to prevent signing that assembly, and
we do this for our build tools assemblies.  The SkipSigning property
can also be set on the command line to prevent signing all assemblies.
Merge branch 'enable-signing'
Removed duplicate string.Format

Console.WriteLine 'includes' a string.Format so the string.Format inside
Console.WriteLine is not needed.
Removed missing files from csproj

In the Microsoft.DotNet.Build.Tasks there were some files which are not
(yet) checked in.
Use separate obj directory per project.

The buildtools were overriding the intermediate path to be the same for every
project. This causes issues because not all intermediate files have unique
names. In particular, incremental or parallel builds could silently put the
resources from a different project in to an assembly!
Update CONTRIBUTING.md to point to our wiki
Adding the appveyor yaml file to setup CI.
Adding appveyor status badge to readme.md
Merge pull request #15 from Sridhar-MS/appveyor-setup

Appveyor setup
Fix for building repository with spaces in root

build.cmd was failing if the repository has spaces in its path.
Merge pull request #16 from Sridhar-MS/space_in_path-fixes

Fix for building repository with spaces in root
Updated link to Strong Naming documentation
Merge pull request #17 from jozefizso/patch-1

Updated link to Strong Naming documentation
Merge pull request #13 from terrajobst/contrib-changes

Update CONTRIBUTING.md to point to our wiki
Fix incorrect link in README.md
Merge pull request #10 from yannisgu/fixCsproj

Removed missing files from csproj
Merge pull request #9 from yannisgu/removeRedundancy

Removed duplicate string.Format
Make xunit runner task take IncludeTraits and ExcludeTraits properties
Add error to build.cmd when running in a non DevEnv command prompt.

Update links to developer guide in error messages to new location
on the dotnet/corefx wiki.
Merge remote-tracking branch 'kr/XunitTraits'