Tolerate . in FS.makeTree

The `normal` function exported by `fs-boot.js` consumes all `.` (so `normal('.')` returns `''`).  If `makeTree` is called with `'.'`, this is reduced to `''` before being passed to `fs.mkdir` (which fails).

It looks like there would be a number of places this could be addressed.  472d23c88f73af206e1d00a5930abb20d6754187 is a minimal change to make `makeTree` tolerant to `'.'`.

I'm running into this case when calling `FS.makeTree(path.dirname(filepath))` (see tschaub/grunt-gh-pages#17).