Support opening folders in Visual Studio and auto detecting P4CONFIG files

Version 2.1 adds the ability to open a folder in Visual Studio and have P4SimpleScc provide source control for it (thanks @inequation). If you open a new solution or folder in Visual Studio that was not controlled by P4SimpleScc before, it will automatically check for a P4CONFIG file, and, if it's valid, P4SimpleScc will automatically be set to use it by default. You can still change the P4SimpleScc setting to 'disabled' or 'manual' if you want to override that. Any P4SimpleScc settings prior to version 2.1 should still apply even if a P4CONFIG file exists and wasn't being used before. Only new solutions or folders will automatically use the P4CONFIG file.

I forgot to mention... currently you can only check out files from folders by modifying them. The right click on the file tab in the editor or in the Solution Explorer doesn't work yet. I'm still investigating what needs to be done there.