Add biplots to make_emperor.py -- fixes #12

A summary of the new features included in this pull request:

- Specify as an input a summarized otu table to create a biplot.
- Specify the name of the output file for the biplot coordinates.
- Specify the number of taxa that need to be kept from the ipnut.
- Show/hide the labels for the taxa elements in the GUI.
- Show specific GUI elements for biplots/jackknifed plots (the ellipse opacity slider will only be presented when the plot contains ellipses).

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This pull request also includes two new command calls in the script usage examples.