Import FASTA and Genbank readers from inputchecker

[necrosovereign]

I'm working on the inputchecker package that will provide functions to collect information about input files a for TaxI and possibly other tools. I think that functionality of inputchecker requires FASTA and Genbank readers that are currently in library/fasta.py and library/genbank.py. So, to prevent code duplication, when inputchecker will become a dependency of TaxI, the readers should also be imported from it.