Rename RemoveStarSequences to RemoveUnknownSequences
only allow 20 known amino acids
Code improvements

- Use + instead of * in the regex so as to not allow empty strings
- Emmit a warning message if a sequence is being filtered
use Genome::Info::CodonToAminoAcid to get list of valid amino acids instead of hardcoding them
create regular expression only once
fixed typos
handle undefined sequence string
preserve encapsulation
Merge pull request #96 from susannasiebert/epitope_prediction

Filter out sequences that contain unknown amino acids