Port flatten over to skbio

This implementation of flatten is much cleaner and faster than other of
the available alternatives.
Fix problems with the Trie documentation

There were a few missing things that needed to be changed to make the
documentation render correctly.

Fixes #170
Fix PEP8 annoyances
Make a few mpl tests more lenient

Fixes #179.
force agg backend in tests

Should remove mpl warning messages for OS X users. Thanks @antgonza for letting me use your laptop to test this!
Merge pull request #181 from ElBrogrammer/issue-179

Pulled branch, tested, looks good
Remove unused import

Thanks @wasade 🍻
Change doc title to Prefix tree
Merge branch 'master' of git://github.com/biocore/bipy into issue-170

Conflicts:
	doc/source/index.rst
Merge pull request #172 from ElDeveloper/port-flatten

Port flatten over to skbio
Add exception type and minor update to doc string
Merge pull request #187 from adamrp/issue_185

Add exception type and minor update to doc string
Merge pull request #173 from ElDeveloper/issue-170

Fix problems with the Trie documentation