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.
Can you update to use the iupac characters in skbio's DNASequence class?

http://scikit-bio.org/generated/skbio.core.sequence.DNASequence.iupac_characters.html#skbio.core.sequence.DNASequence.iupac_characters