Added relative path support for opening files in grunt project folder

Added node "path" dependency in preparation for relative path support.

Added isRelativePath:boolean option allowing grunt-open to launch files relative to Gruntfile.js. 

Use case:
User creates a build of their project and wishes to launch the file in Internet Explorer (ugh I know) using the file:// protocol.  Setting the relative path to true will cause grunt-open to get the full path to the file allowing it to open properly.