Add notebook for filtering

Does NOT use parallel interfaces, so this should probably be updated soon.
However, it does give the general idea.
Add selcet_gamma.py (authored by Amnon)

Used in the filtering notebook.
Add Amnon's latest select_gamma, plus unit tests
Add parallel support using cluster_utils.ipy
Add logic to skip merging if there's only one file
Merge branch 'filtering_notebook' of github.com:adamrp/American-Gut into filtering_notebook
Most recent notebook for filtering

This will likely change soon as we find a better way to find the threshold
that is used to identify contaminant sequences in an OTU.  Currently, the
threshold is set to 90%; if a single sequence composes >=90% of the over-
represented gammaproteo OTU, then the sequence is labeled as a contaminant.
Instead, we would like to define this threshold based on the sequencing
error rate as determined by the Illumina instrument.
Add 100-nt filtering, plus minor revisions
Merge pull request #50 from adamrp/filtering_notebook

[WIP] Filtering notebook [WIP]