Fix compilation when using a simple watch expression. (Closes #11)

Set the output directory to the current working directory when
no `:input` or `:output` option is provided, or the watcher regexp
contains no match group.

lib/guard/coffeescript/runner.rb

@@ -133,7 +133,7 @@ def detect_nested_directories(watchers, files, options)
 
           watchers.product(files).each do |watcher, file|
             if matches = file.match(watcher.pattern)
-              target = matches[1] ? File.join(options[:output], File.dirname(matches[1])).gsub(/\/\.$/, '') : options[:output]
+              target = matches[1] ? File.join(options[:output], File.dirname(matches[1])).gsub(/\/\.$/, '') : options[:output] || '.'
               if directories[target]
                 directories[target] << file
               else