Updates to create a nupkg file

OK I have the build artifacts ready for review. The new files are described below.

 - ```build.proj``` - used to execute the build
 - ```build.ps1``` - script to easily invoke the build script, this is mostly for developer convenience
 - ```getnuget.targets``` - used to get nuget on demand

The build.ps1 is really for developer convenience, for the build server your better off building the ```build.proj``` directly.

#### Usage
You can just invoke ```build.ps1```. You can pass in ```Configuration``` and ```Targets```. The output will be placed in OuptutRoot by default.

I'd recommend creating the folder ```C:\Temp\Nuget\LocalRepo\``` (controlled by the ```NugetLocalRepo``` property) on your local box. When you run the build script the produced .nupkg file will be copied to that folder. This makes it easy to test this because in Visual Studio you can configure a new local feed to point to ```C:\Temp\Nuget\LocalRepo\``` and then you can install the package.

I've marked the package a prerelease so you'll need to add the ```-pre``` flag on the Install-Package command.

```Install-Package markdownlog -pre```

#### CI usage
For CI there will be some other process that builds the assemblies so it would be good we can configure this script to pick up those binaries instead of re-building the. I haven't tested from CI but I think you can configure your CI in the following ways.
 1. Build the .sln file
 1. Build the .proj file passing in properties to skip the build

To skip the build instead of building ```FullBuild``` you should build ```CreateNugetPackage```. I think you'll need to pass the following properties as well
 - ```MarkdownLogBin``` - path the the output folder for Markdown project
 - ```OutputRoot``` set this to a unique value for build.proj