re-defined iupac_characters as combination of new values, iupac_standard_characters and iupac_degenerate_characters; added dict of iupac_degeneracies; fixes #74
modified iupac_*character strings to be lists

this makes a lot more sense for look-ups
addressed comment from @Jorge-C
Merge pull request #79 from gregcaporaso/seq-updates

made iupac_standard_characters, iupac_degenerate_characters, iupac_characters, and iupac_degeneracies static member variables