ENH: Compute engine architecture for FFC API.

This patch lays the groundwork for a compute engine designed to
facilitate construction of factor-based universe screening and portfolio
allocation.  It contains:

A new module, `zipline.modelling`, containing entities that can be used
to express computations as dependency graphs.  Each node in such a graph
is an instance of the base `Term` class, defined in
`zipline.modelling.term`.  Dependency graphs are executed by instances
of `FFCEngine`, defined in `zipline.modelling.engine`.

A new module, `zipline.data.ffc`, containing loaders and dataset
definitions for inputs to the modelling API.

New `TradingAlgorithm` api methods: `add_factor`, and `add_filter`.
These methods can only be called from `initialize`, and are used to
inform the algorithm that each day it should compute the given terms.
Computed factor results are made available through a new attribute of
the `data` object in `before_trading_start` and `handle_data`.  Computed
filter results control which assets are available in the factor matrix
on each day.

setup.py

@@ -24,6 +24,21 @@ ext_modules = [
         ['zipline/assets/_assets.pyx'],
         include_dirs=[np.get_include()],
     ),
+    Extension(
+        'zipline.lib.adjusted_array',
+        ['zipline/lib/adjusted_array.pyx'],
+        include_dirs=[np.get_include()],
+    ),
+    Extension(
+        'zipline.lib.adjustment',
+        ['zipline/lib/adjustment.pyx'],
+        include_dirs=[np.get_include()],
+    ),
+    Extension(
+        'zipline.data.ffc.loaders._us_equity_pricing',
+        ['zipline/data/ffc/loaders/_us_equity_pricing.pyx'],
+        include_dirs=[np.get_include()],
+    ),
 ]
 
 setup(