ENH/BUG: Modeling API enhancements.

- Fixes an error where Modeling API data known as of the close of `day
  N` would be shown to algorithms during `before_trading_start` as of
  the close of the same day.  Algorithms should now only receive data
  during `before_trading_start/handle_data` that was known as of the
  simulation time at which the function would be called.

- All Term instances now have a `mask` attribute that must be a `Filter`
  or an instance of `AssetExists()`.  `mask` can be used to specify that
  a Factor should be computed in a manner that ignores the values that
  were not `True` in the mask.

- Changed the interface for `FFCLoader.load_adjusted_array` and
  `Term._compute` from `(columns, mask)`, with mask as a DataFrame, to
  `(columns, dates, assets, mask)`, where mask is a numpy array.  This
  is primarily to avoid having to reconstruct extra DataFrames when
  using masks produced by non `AssetExists` filters.

- Adds `BoundColumn.latest`, which gives the most-recently-known value
  of a column.

setup.py

@@ -64,8 +64,12 @@ ext_modules = LazyCythonizingList([
     ('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']
+        'zipline.data.ffc.loaders._equities',
+        ['zipline/data/ffc/loaders/_equities.pyx'],
+    ),
+    (
+        'zipline.data.ffc.loaders._adjustments',
+        ['zipline/data/ffc/loaders/_adjustments.pyx'],
     ),
 ])