ENH: Factor out API methods. Add support for algo scripts.

This is a step towards the goal of uniting Quantopian scripts
and zipline.

To make the syntax of zipline identical to Quantopian
we break out the API methods (like order) and turn them into
functions. To access the algo object we add a thread local reference
to the current algorithm that is accessed in the API functions.

TradingAlgorithm now takes either a string or two functions
(initialize and handle_data) that it executes.

Use api method decorator for methods available in algoscript.

Ported appropriate algorithm tests from internal code.