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

Conflicts:
	doc/source/index.rst
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