alt+down no longer enters the options list; pill close buttons receive label markup when created;
adds helper-function addPillLabels to the prototype
updates component dependency names; adds example; refactors #addPillboxLabels;
adds log message for debugging
removes #addPillLabels; instead, clobbers Pillbox#add to include a11y markup
adds aria roles to ul and li elements
comment out input-value population for testing
adds #announce; calls #announce when enter is pressed;
calls #announce when an item is deselected
Merge pull request #1 from isner/master

a11y improvements