Updated NancyContext.ModelValidationResult

It will not use ModelValidationResult.Valid, instead of newing up a
blank result, when no result has explicitly been assigned to the
property.
Fixed XML comment of ModelValidationDescriptor
Updated ModelValidationResult and ModelValidationError

Instead of errors being represented by a simple
IList<ModelValidationError> structure, it is not stored in an
IDictionary<string, IList<ModelValidationError>> structure. The key is the
name of the property that the error represents.

The error message formatter has also been updated from Func<string,
string> to Func<IEnumerable<string>, string> since MemberNames can contain
multiple values

Last, the ModelValidationError can now implicitly be cast to a string
Fixed broken tests
Added missing XML comment
Changes the ModelValidationError signature

It now takes a string as the error message, instead of a
Func<string>. Each validation adapter is now resposible to
format the error message instead of providing a formatter.

This change makes it easier to use for custom errors.
Added ModelValidationResult.Error extension
Moved extension to the root namespace
Saved project :)
Merge pull request #1318 from thecodejunkie/modelvalidationresult-improvements

Updated ModelValidationResult and ModelValidationError