implement custom workflow for assigning taxonomy

[jairideout]

A custom workflow needs to be implemented to easily allow us to assign taxonomy using different taxonomy assigners and/or parameters, as well as easily support/scale to more taxonomy assigners and Illumina runs besides the ones we currently have.

[jairideout]

I wrote the initial code for this script, which included support for the RDP classifier and multiple datasets. @zellett and @kpatnode added support for blast, mothur, and rtax, as well as timing output for each assigner and support for efficiently rerunning the script over an existing output directory.

Closing as the framework is in place; we should be able to easily add support for other taxonomy assigners in the future as needed.