Add getBindingType() method in DecimalType

Hi,

I was trying to use the findBy method from the repository, where some of the criteria data were negative floats values. I didn't get any results, though I had coincidental records in the BD with that criteria.

After searching I found this issue: http://stackoverflow.com/questions/8300208/php-pdo-prepare-negatives-become-positive

So, it seemed the problem was caused for the same reason. I've added the getBindingType() returning the PDO::PARAM_INT constant and now my findBy is returning the results correctly.