Refactor: unify downloader & wind Dataset classes

The primary goal was to have one (instead of two) Dataset classes. To
achieve this,

  - There is now a more general Dataset class that can open files, has a
    few utility functions to list files and so forth (from the original
    Downloader Dataset class). It will mmap the file, but do no more.
  - The downloader will construct a numpy array from the mmap by passing
    it as a buffer to ndarray(...)
  - The interpolation code will cast the mmap to a double array once
    (keeping the result in a closure), producing a function that can be
    used to get wind u/v (from the original wind.pyx)

Along the way,

  - The README was updated to reflect current reality
  - The tawhiri-download script is now produced by setup.py
  - A Dataset.find_recent method was added (that does what it says on
    the tin).