MAINT: Perspective offset for load adjustments.

Add a perspective offset to `AdjustedArrayWindow` and `AdjustedArray`,
so that `HistoryLoader` does not need to twiddle with offsets to support
viewing the data from the bar after end of the window, (Which is the
case when a '1d' history window is retrieved in minute mode, which is
explained in the docstring for `HistoryLoader.history`)

Presently, this simplifies the logic in
`HistoryLoader._get_adjustments_in_range`, and other incoming
AdjustmentReader's, (e.g. the roll based adjustment reader for continous
futures.) This patch should also make it easier for history and pipeline
to converge on a singular `load_adjustments` method.

zipline/lib/adjusted_array.py

@@ -152,16 +152,22 @@ class AdjustedArray(object):
     missing_value : object
         A value to use to fill missing data in yielded windows.
         Should be a value coercible to `data.dtype`.
+    perspective_offset : int
+        The number of rows after the current end of the window, from which the
+        data is being viewed. This value is used so that adjustments that occur
+        between the end of the window and the vantage point are applied.
     """
     __slots__ = (
         '_data',
         '_view_kwargs',
         'adjustments',
         'missing_value',
+        'perspective_offset',
         '__weakref__',
     )
 
-    def __init__(self, data, mask, adjustments, missing_value):
+    def __init__(self, data, mask, adjustments, missing_value,
+                 perspective_offset=0):
         self._data, self._view_kwargs = _normalize_array(data, missing_value)
 
         self.adjustments = adjustments
@@ -177,6 +183,8 @@ class AdjustedArray(object):
                 )
             self._data[~mask] = self.missing_value
 
+        self.perspective_offset = perspective_offset
+
     @lazyval
     def data(self):
         """
@@ -220,6 +228,7 @@ class AdjustedArray(object):
             self.adjustments,
             offset,
             window_length,
+            self.perspective_offset
         )
 
     def inspect(self):