Add six-frame translation recipe

@ebolyen and I worked on this. Thanks @gregcaporaso for the great suggestions along the way!

Fixes #29.
Add new six-frame translation recipe to Index.ipynb
updated some of the text and variable names for accuracy
Merge pull request #31 from gregcaporaso/issue-29

new recipe of the week
ENH: addition of pd and unweighted unifrac code, ported from IAB
first full pass at new recipe
ENH: added figure
STY: cleaned up text
STY: text clean-up
ENH: added new chapter to TOC
ENH: added new function at bottom
Merge pull request #33 from gregcaporaso/issue-32

added recipe on computing phylogenetic diversity metrics