Add SI prefix option to Numeric bytes core ext

This allows the use of SI instead of binary prefixes and introduces
symmetry between the Numeric core ext and AS::NumberHelper.number_to_human_size.

Please see http://physics.nist.gov/cuu/Units/prefixes.html and
http://physics.nist.gov/cuu/Units/binary.html.

before:

  1.kilobyte # => 1024
  NumberHelper.number_to_human_size(1.kilobyte, prefix: :si) # => "1.02 KB"

after:

  1.kilobyte                 # => 1024
  1.kilobyte(:si)            # => 1000
  1.kilobyte(:prefix => :si) # => 1000
  NumberHelper.number_to_human_size(1.kilobyte, prefix: :si) # => "1 KB"