Added support for enum values in expressions

Tried an expression like:

``` C#
[RequiredIfExpression(Expression = "!{0}", DependentProperties = new[] { "Schedule" }, TargetValues = new object[] { AgreementSchedule.Custom }, ErrorMessage = "required for non-custom schedules.")]
```

Looks like the attribute renders a numeric comparison, but the dependent property gives a string.

I fixed by converting to numeric if both sides are numeric.