Reference OrleansSDK using $(OrleansSDK) property.

Currently the project assumes a directory layout which may not hold for some users. Instead, I propose we use the $(OrleansSDK) property, which is used in the file copy process.

For systems which don't have the Orleans SDK installed (eg, build servers), the SDK can be included in the solution directory and referenced by including a small MSBuild file.

IncludeOrleansSDK.targets:
```
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
	<PropertyGroup>
		<OrleansSDK>$(MSBuildThisFileDirectory)\Dependencies\Orleans\SDK</OrleansSDK>
	</PropertyGroup>
</Project>
```