Pocket trawler

Not all functionality implemented. But, it is known to work under certain conditions.

Will:

1. Take a list of species
2. Take a downloaded copy of the assembly_summary file
3. Will check for Aspera and Kraken dependencies
4. Will produce a Kraken DB as long as all the genomes have a GI number

Known bugs:

1. Will not exit nicely if a genome does not have a GI number
2. If the user does not nominate a kraken_db name, it will automatically choose None, which will ultimately cause it to fail

Known issues:

1. NCBI does not issue GI numbers any longer. Will have to wait for Kraken to find another way of mapping genomes to the taxonomy tree. It is being worked on.
2. Documentation still lacks an example.