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.  83a58047d18e50ebc268c92f94820658d83a8be1 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).