Integrate PR from @rschiefer with fixes.

This PR is based on https://github.com/davidwhitney/ReallySimpleFeatureToggle/pull/1
It implements multi-tenancy in the MVC plugin.

To configure the MVC plugin you must call: 

            ReallySimpleFeature.Toggles.Configure
                .WithPlugin(new WebPlugin(persistRandomPercentageResultToCookie:true))
                .WithPlugin(new MvcPlugin());

Now, to lock down the plugin for a specific tenant you can call:

            ReallySimpleFeature.Toggles.Configure
                .WithPlugin(new WebPlugin(persistRandomPercentageResultToCookie:true))
                .WithPlugin(new MvcPlugin(()=>"MyLockedTenant"));

Alternatively, you can provide a method that retrieves the tenant for the current web request (recommended):

            ReallySimpleFeature.Toggles.Configure
                .WithPlugin(new WebPlugin(persistRandomPercentageResultToCookie:true))
                .WithPlugin(new MvcPlugin(()=>SomeMethodCallThatReturnsAString));

Or provide an implementation of the interface:

    public interface IRetrieveTheTenantForThisRequest    {
        string GetCurrentTenant();
    }

like:
            ReallySimpleFeature.Toggles.Configure
                .WithPlugin(new WebPlugin(persistRandomPercentageResultToCookie:true))
                .WithPlugin(new MvcPlugin(new MyFactoryThatImplementsIRetrieveTheTenantForThisRequest() ));

All MVC html helpers will then return configurations based on the tenant configured. Omitting this configuration will return the configuration for Tenant.All.