Modified check_id_map.py to use IUPAC characters from bipy, issue #1421
Updated import statement from bipy to skbio to match renaming
Modified DNASequence.iupac_characters call in qiime/check_id_map.py per Jai's request #1422
Fixed one other call to IUPAC DNA char functions for #1422
Modified per Jai's comments #1422
Merge pull request #1422 from walterst/validate_mapping_file_bipy1421

Modified check_id_map.py to use IUPAC characters from bipy, issue #1421