Validator moves to it's own package

Today I move the code of LaTrompa.Validation into it’s own package, with no external dependencies (besides the framework of course).

I added a class to validate XmlNodes.

The string validation now has a IsOneOf method to match the string against a list of options.

I still need to add a generic version for this method, I guess it pass by me.

Also in the string validation there are two new validations to check if a string can be converted into an Int32 or a Boolean.

The whole conversion validation looks like a good candidate for refactoring and generalize it into a base class. that way it will be easy to offer conversion checking against all types. I added the ThrowFirst method into the Validator class. This method was available only in the CheckThat classes.

I moved all the Throw methods into their own class ErrorThrower (I should rename this to ExceptionThrower). Previously I was doing almost the same thing in two different places. Not very DRY.