Handle case when no search term is passed in

When ctrlp opens, it runs matcher with no search term.
Matcher was not correctly handling this and was
setting the search term to the last command line arg.
Also stopped filtering of zero score items when no
search term is passed. This caused weird behaviour in ctrlp.
Fixes issue #14
Merge pull request #15 from mleech/master

Handle case when no search term is passed in. (Fix for issue #14)