ENH: Create Emperor object

This object will allow users to display PCoA plots from within the
IPython notebook.
DOC: Add documentation to Emperor object
API: Add top level availability of Emperor
ENH: Add docstring usage example
TST: Add tests for emperor.core
ENH: Add Emperor object to emperor.core

Fix #268
Added capital E to valid letters

Filename couldn't be saved with an uppercase E because it was not listed as a valid code. This resolves this.
Removed the extra 68 to allow effective 69.
Added note to changelog.
Much desired 69 now available.

[skip ci]
Merge pull request #284 from wdwvt1/missing_69

Added capital E to valid letters
Merge pull request #281 from ElDeveloper/core

ENH: Add Emperor class