Fixes #38: Don't let async.forEachSeries overrun

asyncforEachSeries' continue() callback must be the last executed
statement or it will iterate past the last item in the collection.

This will then cause 'f' to be undefined in that iteration.
Warn if no source files are provided
Merge pull request #39 from jasonkarns/gracefully_handle_no_sources

Gracefully handle no sources