Added missing installation package and changed build to .NET 4.5

In relation to this [issue](https://github.com/Sitecore/Sitecore-MarkdownField/issues/1), I made the necessary changes in my forked version of the solution. These changes include updating the project to build for .NET 4.5 and adding an installable package, which contains all required resources (and thereby not requiring to build the package beforehand).