Also check for .tags

This PR would theoretically make the package scan for `.tags` as a possible source in addition to `tags` and `TAGS`. It'd be useful to have the ctags file hidden to make the root a bit cleaner. (Also, convention in the Sublime Text [CTags package](https://github.com/SublimeText/CTags) is to put the ctags file as `.tags`)

I use "theoretically" since the PR doesn't work yet. By modifying the package in Atom.app itself, adding:

      tagsFile = atom.project.resolve(".tags");
      if (fs.isFileSync(tagsFile)) {
        console.log('hi');
        return tagsFile;
      }

It looks like it can find `.tags`, since "hi" shows up in the console, but for some reason the popup still says "Project has no tags file or it is empty". Trying to work with the package in dev mode triggers one of these:

    Uncaught Error: Cannot find module 'pathwatcher' <- When pressing cmd+shift+r
    Uncaught Error: Cannot find module 'fs-plus' <- When pressing cmd+r, alt+cmd+up
    Uncaught Error: Cannot find module 'q' <- When pressing alt+cmd+down