Add NuGet package (fixes #1)

This specs out a NuGet package build, rebuilt automatically on every build in Visual Studio. I've also built the current release and pushed it to the NuGet Gallery: https://www.nuget.org/packages/JSErrorCollector

When this is installed into a project the package installs the DLL in ```$(SolutionDir)/packages/JSErrorCollector.0.5.1.0/lib/net40/JSErrorCollector.dll``` and adds a reference to that in the target project, and then puts JSErrorCollector.xpi in ```$(SolutionDir)/packages/JSErrorCollector.0.5.1.0/JSErrorCollector.xpi```

I'll email you the username and password for the JSErrorCollector NuGet account, so you can push releases there yourself.