Add sortmerna to pick_otus.py

This merge makes the following additions to pick_otus.py:

1. All the necessary commands for OTU-picking using SortMeRNA (in /scripts/pick_otus.py)
2. A new SortMeRNAV2OtuPicker class for indexing a database (if not provided) and running SortMeRNA (in /qiime/pick_otus.py)

The corresponding application controller + unit tests added to brokit.
The file handler of `open(seq_path, 'U')` is never closed. You can use context managers