Normalize relative path for document-root on Windows

- Calling resolve() doesn't fully normalize relative path on Windows (in Node v0.11.x, see
  joyent/node#7031 ), i.e. doesn't lowercase drive letter, while normalize() does, which causes
  paths resulting from calling normalize() and join() (depending on normalize()) to differ and to
  break comparisons.

Fixes cloudhead/node-static#42.