Issue #79 IFieldConfiguration.AddFieldContainerClass

Hi MRCollectives,

I've added in the AddFieldContainerClass method as suggested for this #upforgrabs issue. Please feel free to suggest a different approach if you can think a more elegant one. Hopefully I have set up the tests correctly and the implementation is sound. Whatever the case it was a great learning experience for me!

Note: I committed the  ChameleonForms.Tests package config and project file changes from adding the NUnit Test Adapter nuget package to run the tests locally. I should not have committed those but it's probably easy for you to ignore these changes during the merge right?

Cheers!