Added an 'Empty' choice for deck slots which aren't in use

This might fix some bugs like #404 and #408 because previously slots could internally have no deck assigned, but the UI would show that deck as being filled because there was no way to display an unused deck slot. In other words, you might think you had a deck assigned to the slot, but in fact nothing was... now at least you'll know there's no deck assigned.

(cherry picked from commit 74f4ee4)
Released v0.18.2
add code coverage plugin
try integration with Coveralls service
only cover for osx
switch from jacoco to cobertura to handle aggregate multi module report
add code coverage badge
Update README.md
Update README.md
update code coverage badge
fix #419 : merged labels prevented match popup creation
format + organize imports
Merge branch 'master' of https://github.com/tyrcho/HearthStats.net-Uploader

Conflicts:
	README.md