uclust picker handles filenames with multiple dots

Intermediate .uc files are now correctly named based on the input sequences filename, including the case where there are multiple dots in the filename.

Also updated QIIME codebase to use uclust.py in pycogent_backports/ (was previously only being used in assign_taxonomy.py).

Fixes #191.
Merge pull request #1191 from jrrideout/issue-191

uclust OTU picker handles filenames with multiple dots