Added an exporter for skills required for a given fit

This work adds a menu item to export the skills necessary for the fit shown in one of three formats: an EVEMon-compatible EMP file, EVEMon-compatible XML file, and a text file.

It is based on the same data used to construct the tooltip for the skills icon to the right of the Character field on the main screen.

In addition to satisfying request PYFA-12 (http://jira.evefit.org/browse/PYFA-12), this also makes it far easier to examine large skill trees for complex fits.