BUG: Put initialization of perf_tracker back in __init__

The initialization of perf_tracker had been moved from __init__
in TradingAlgorithm to _create_generator. This caused perf_tracker
to not be ready when portfolio requested it. portfolio was consequently
not ready for access in init. portfolio can now be accessed in init
again, assuming valid sim_params are passed. Otherwise it will be
available in handle_data() after _create_generator() is called.
Merge pull request #372 from quantopian/moved-perf-tracker-assignment

BUG: Put initialization of perf_tracker back in __init__
BLD: Add Python as a conda dependency.
BUG: Fix comparison for pandas 0.14. Closes #377.
Bump version: 0.7.0rc1 → 0.7.0
BLD: Update md5 sum in meta.yaml.
DOC: Fix readme example and update dependency doc.
BUG: Fixed random dips in returns as shown to user.

Previously the last sale price was not correctly being set on
positions when the transaction arrived before the trade event.
The last sale price was defaulted to zero and never updated. This resulted
in one holding stocks that were bough >>0 and now had value 0 from
the perspective of returns. The returns would display correctly again
when the next trade of that security happened. For most securities trading is
frequent enough that there's no issue, but for some illiquid ones it took
hours to fix itself.

Updated test_perf_tracking:TestPerformanceTracker.test_minute_tracker
This test was based on assuming that last_sale_price was zero,
allowing the sharpe ratio to be calculated. The sharpe ratio can no longer
be calculated for this specific tested scenario and the test has been changed
accordingly.
Merge pull request #378 from quantopian/fixed-zero-last-sale-price

BUG: Fixed random dips in returns as shown to user.
Created 0.7.1 release notes
DOC: Updated dual moving average example to use symbol lookup.
MAINT: Factor talib tests out to separate file.
MAINT: Make talib an optional dependency. Fixes #362.
MAINT: Log more information on history update failure.

Log the field and algo datetime when the update of the return frame
fails; re-raise the error until the when and why of the value error is
understood.
Merge pull request #382 from quantopian/talib_optional

Make talib an optional dependency.
ENH: Add new order statuses for broker integration
BUG: Clean up handling of reject() and hold() orders
TST: Add tests for behavior of rejected/held orders.

Also made a tweak to the handling of Order.status
for when a held order is filled (partial or full).
ENH: Re-work the usage of _status on Order
Merge pull request #380 from quantopian/broker_order_statuses

ENH: Add new order statuses for broker integration