Visual Studio Project for Open XML SDK

Created a Visual Studio project (using Visual Studio 2010) that is configured to build the Open XML SDK like the PowerShell and makefile-based build process. The project .csproj file contains manual edits to make sure that the .resources files are put in the right namespace. The project contains a pre-build command line that launches a PowerShell script based on the existing BldSdk.ps1 script to set the version string in the AssemblyInfo.cs. 

Changed .gitignore to also ignore .resources files.

Edited the README.md file to describe the Visual Studio-based build process, which also requires the configuration of the execution policy.

Added the SetAssemblyVersionString.ps1 PowerShell script which is used as a pre-build script. It reuses some code from BldSdk.ps1. However, the version string is computed without using the minute to make sure Debug and Release DLLs built at least during the same hour (e.g., in a batch build) have the same version string.