Merge pull request #17 from martinghunt/master

Tidy up checking external programs OK
added kraken.download_from_genbank()
add kraken.download_extra_refs()
Convert extra ref files from gb to embl
Rewrite kraken module to be a class
Check success of downloading from ncbi; don't add same line more than once to kraken files taxonomy files
Only print gbk filenames when being verbose
Write reference genome files info to file
Keep embl_dir as absolute path
Subsample to 40x instead of 20x