rename tabs

this addresses issue #77. i also removed the alpha rarefaction tab, and will put that information in a new section on the alpha diversity boxplots page.
adding back alpha rarefaction plots

linked under an 'advanced' section in alpha diversity boxplots.
worked on text for alpha diversity

this addresses #70. #77 was addressed in previous commit, but forgot to note the issue number.
Merge branch 'master' into text-updates
fixed two typos
Merge pull request #87 from gregcaporaso/text-updates

Text updates