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__