rna: Ditch first example.
rna: Extend example.

Forward ==, to_str, to_s.
Extract NucleicAcid baseclass.
Add RibonucleicAcid class.
rna: Extend tests. Use long classnames.
Merge pull request #332 from pck/extended-rna-transcription-test-v2

Extended rna transcription test v2