Set Campaign confidence to 'low' if unexpected value is provided

It seems that this base function should enforce the three "low", "medium", or "high" options for Campaign Confidence. If the provided value is not one of those three, the code should either throw an error, or set the value to the default of "low".

I went with the latter option in this case. What do you think?
Merge pull request #330 from brlogan/patch-3

Set Campaign confidence to 'low' if unexpected value is provided