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
Merge pull request #281 from ElDeveloper/core

ENH: Add Emperor class