ENH: Add adjusted history for continuous futures.

Add `.adj('mul')` and `.adj('add')` methods on ContinuousFuture, which
when used with `history`, will calculate and apply adjustments so that
the values are adjusted to account for discounts and premiums during
rolls.

Example usage in an algo:

```
from zipline.api import continuous_future

def initialize(context):
    context.cl_add = continuous_future('CL', offset=0, roll='calendar').adj('add')
    context.cl_mul = continuous_future('CL', offset=0, roll='calendar').adj('mul')
    context.cl = continuous_future('CL', offset=0, roll='calendar')
    schedule_function(print_history)

def print_history(context, data):
    frame = data.history([context.cl, context.cl_add, context.cl_mul],
                         ['price', 'sid'],
                         20,
                         '1d')
    print 'unadjusted'
    print frame.loc[:, :, context.cl]
    print 'adjusted add'
    print frame.loc[:, :, context.cl_add]
    print 'adjusted mul'
    print frame.loc[:, :, context.cl_mul]
```

zipline/data/continuous_future_reader.py

@@ -229,8 +229,6 @@ class ContinuousFutureMinuteBarReader(SessionBarReader):
         # Get partitions
         partitions_by_asset = {}
         for asset in assets:
-            rolls_by_asset[asset] = rf.get_rolls(
-                asset.root_symbol, start_date, end_date, asset.offset)
             partitions = []
             partitions_by_asset[asset] = partitions
             rolls = rolls_by_asset[asset]