addition of LEA-Seq method

Implements Low-Error Amplicon Sequencing (LEA-Seq) method, described in: Faith, Jeremiah J., et al. "The long-term stability of the human gut microbiota." Science 341.6141 (2013).
I need to add a couple of functions in split_libraries_lea_seq.py, and I need to add unit tests. I will add those soon.
In the function, I am calling uclust a large number of times. Initially I was using brokit.get_clusters_from_fasta_filepath function, but due to issue (https://github.com/biocore/brokit/issues/31), I have temporarily reverted to qiime_system_call.