Use fs.watch instead of fs.watchFile

This pull request modifies node-tail to use `fs.watch` instead of `fs.watchFile`, which is now [deprecated](http://nodejs.org/api/fs.html#fs_fs_watchfile_filename_options_listener).  It follows issue #6.
* Tail.{watch,unwatch} have been moved to Tail.{watch,unwatch}File
* Tail gains two new instance variables, @pos and @watcher for newly required book-keeping
* readBlock / buffer / queue internals have been mostly untouched, so if you want to implement a graceful fallback, it shouldn't be too hard. 
* however, I am unsure how to detect whether or not `fs.watch` is available. The [docs](http://nodejs.org/api/fs.html#fs_availability) seem to indicate it just "won't work", without really describing what that looks like.
* I have not bumped any version numbers, changed the API, or done anything else besides substituting the underlying watch implementation and testing it