Refactor PHPathWatcher to take an array of paths to watch

As suggested by @sdegutis, see
https://github.com/jasonm23/phoenix/commit/1fc01d51ea2cad73c263433c3e38c56d19849ae3#commitcomment-7501288
Merge pull request #5 from pascalw/refactor-path-watcher

Refactor PHPathWatcher to take an array of paths to watch