MAINT: Reverses polarity on keep transactions default.

So that transactions are kept by default.

This prepares for the addition of the serialize flag added by
@fawce.

Setting the default to True, so that the flags will be aligned.
ENH: Adds a flag for optionally not serializing positions.

So that both computational and memory overhead is reduced,
this turns off serializing positions for cumulative performance.

Positions were essentially being doubled up by being stored
in both cumalative and daily.
MAINT: Updates pep8 from 1.4.4 -> 1.4.5
MAINT: Uses copysign to extract direction of order.

Instead of using division of the amount by itself to extract
the direction, uses math's copysign.

Should be almost functionally equivalent,
but copysign won't have a possible floating point error leading
the direction to not be exactly 1.
MAINT: Updates copyright date of slippage module.
MAINT: Accounts for possible floating point rounding in slippage.

Uses numpy's allclose instead of straight comparison of floating
points, so that there is some tolerance of floating point rounding.
MAINT: Uses copysign to extract order direction in FixedSlippage.

Uses copysign instead of division by fabs.
MAINT: More slippage comparison changes for floating point.

Continues conversion of floating point comparisons in slippage to
use numpy's allclose.
BUG: Fixes check reversal in slippage transact_stub.

Previous commit accidentally flipped the check when converting
to use `allclose`.
MAINT: Removes unneeded assignment in slippage.

`direction` is set on each iteration of the loop,
making the initial assignment unneeded.
REF: Import interface.

Makes TradingAlgorithm available at the top-level.
So that algorithms can do:
```
import zipline

class MyAlgo(zipline.TradingAlgorithm)
```

OR

```
from zipline import TradingAlgorithm

class MyAlgo(TradingAlgorithm)
```
MAINT: New pytz version in requirements.txt
MAINT: Defines members of the Order object.

Instead of a loosely defined object for Order, explicitly
defines the parameters and corresponding members.

Clearing the way for adding more members to the Order object.
MAINT: Updates copyright for recently changed files.
MAINT: PyPI updates: six==1.3.0, delorean==0.2.0
MAINT: added typed errors module

- added exceptions in place of asserts for expected fields for rolling
transforms.
- removed assertions with Messages in favor of typed exceptions.
ENH: added a CUSTOM datasource type for custom data.

- perf modified to let non-performance related events flow through.
- changes to support streaming non-trading data through batch transforms
and for mixing in sids with just custom data.
- allowing CUSTOM events to flow through to transforms.
- Added logic to maintain pre-specified sid filter.
ENH: Adds new order types.

Adds the following order types:
- market
- limit
- stop
- stop limit
MAINT: Removes ipython related requirements.

Removes from requirements_dev.txt, since requirements_dev.txt is
used in various unit testing environments, ipython, pyzmq, etc.
is extra weight that is being unecessarily pulled in.
STY: Removes unused import.