Add support for comparison operators

This includes a few simple changes required to support comparison operators such as < and >. A minor tweak was made to the select statement tweaker since the node created by a statement such as the following does not respond to :children.

    t = arel_table
    where(t[:numeric].gt(1))

In addition, since postgres does not perform implicit typecasting from string to integer when < is used, an optional column_type parameter is used on the pgcrypto class method. The value given to it is directly appended to the select statement to perform typecasting.