Create Umbraco.gitignore

A git ignore file for [Umbraco](http://www.umbraco.com/) development. Umbraco's source can be found on [Github](https://github.com/umbraco/Umbraco-CMS/). The requested change is being made as most, if not all Umbraco developers should not place certain Umbraco generated folders and files into source control. This applies to some Visual Studio generated files too.