Add reverse-complement function

[maasha]

Will require that we know the sequence type which must be either DNA or RNA.

Hint:

      if self.is_dna?
        entry.seq = self.seq.tr('AGCUTRYWSMKHDVBNagcutrywsmkhdvbn', 'TCGAAYRWSKMDHBVNtcgaayrwskmdhbvn')
      elsif self.is_rna?
        entry.seq = self.seq.tr('AGCUTRYWSMKHDVBNagcutrywsmkhdvbn', 'UCGAAYRWSKMDHBVNucgaayrwskmdhbvn')
      else
        raise SeqError, "Cannot complement sequence type: #{self.type}"
      end