Improve documentation expressions for validate_numericality_of

Hi, this PR is intended to generate better documentations for validate_numericality_of matcher.
This was originated from #413 which was intended to fix just ComparisonMatcher#allowed_types.
I also found other parts which seem to have some room for improvements while I was working on this, but they are for next times :)

Main changes are:
- NumericalityMatchers are categorized NumericTypeMatchers and ComparisonMather
- Integer, Odd, Even matchers inherit NumericTypeMatcher which respond to allow_type
- ComparisonMatcher does not respond to allowd_types anymore.

Thanks!