ENH: Improvements to rank().

- Add an `ascending=True` keyword to `rank()`.

- Add `top(N)` and `bottom(N)` methods to Factor.  These return Filters
  that pass the top and bottom N elements each day.

- Add a slightly faster path for rank(method='ordinal').  I had
  originally thought the fast path was 2-3x faster because I had my
  benchmark data axes flipped.  The actual speedup is only 5-10%, which
  means it probably wasn't worth the effort to Cythonize...but we have a
  slightly faster version now so we might as well use it.

- Refactor test_filter and test_factor to make it easier to implement
  and test transformations on factors.  These tests now subclass
  BaseFFCTestCase, which provides facilities for passing a dict of terms
  and an "initial_workspace", the values for which are used by
  SimpleFFCEngine rather than needing to manually manage the inputs and
  outputs of each term.

setup.py

@@ -62,6 +62,7 @@ ext_modules = LazyCythonizingList([
     ('zipline.assets._assets', ['zipline/assets/_assets.pyx']),
     ('zipline.lib.adjusted_array', ['zipline/lib/adjusted_array.pyx']),
     ('zipline.lib.adjustment', ['zipline/lib/adjustment.pyx']),
+    ('zipline.lib.rank', ['zipline/lib/rank.pyx']),
     (
         'zipline.data.ffc.loaders._us_equity_pricing',
         ['zipline/data/ffc/loaders/_us_equity_pricing.pyx']