nucleotide-count: Model nucleotides as bytes

Unlike other languages, Go has two datatypes for single letters: rune
and byte. As our abbreviations for the nucleotides, A, C, G and T, can
all be ASCII-encoded, multibyte runes aren't needed.
Merge pull request #36 from sjakobi/nucleotide-count-bytes

nucleotide-count: Model nucleotides as bytes