API/DOC/TST: added, TreeNode.cache_attr, docs for it, tests for
TreeNode.invalidate_caches(), and let invalidate_caches optionally invalidate
cached attributes
ENH: cache_attr allows frozenset now
TST/ENH/MAINT/DOC: decomposed, cleaned up, added tests, added docs
Merge branch 'master' of github.com:biocore/scikit-bio into issue422

Conflicts:
	skbio/core/tree/tests/test_tree.py
BLD: py33 support on a dict_values return
added majority rule to init, thanks @gregcaporaso\!
BLD: py3 warnings
BLD: py3 warnings
DOC: addressing @gregcaporaso's comments
BUG: cutoff threshold should be < not <=
TST/PY3: multiple tree test and fixing py3 annoyances
DOC: added multitree example
DOC: adding Matthew Wakefield to contributors
DOC: added majority rule to init
STY: pep8
DOC: adding @genomematt
Merge pull request #473 from wasade/issue422

Issue422