DOC: Add document describing file formats

I have added a new RST document that describes the file formats Emperor
uses to display plots. Classic coordinate files (as generated by QIIME
<= 1.8.0), scikit-bio coordinates files and QIIME metadata mapping files
are all to some extent described here.

Fixes #286
DOC: Rephrase documentation
DOC: Add note to ChangeLog.md about format docs
DOC: typos and other corrections

Thanks @antgonza
Merge pull request #302 from ElDeveloper/issue-286

DOC: Add document describing file formats