Added application controller framework for usearch6.1 (open and closed reference OTU picking).  Modified usearch 5.X framework to work like uclust_ref for OTU matches and failures for future parallelization.
Merge branch 'pick_otus_usearch_rev' of git://github.com/walterst/qiime into issue-833
add usearch61_ref as a supported method for closed-reference otu picking
renamed for consistency with method name
updated class name and fixed bug that caused intermittent failures