added LastTradeVolume observable
Refactoring observables using external libraries (pandas, ta-lib)
cleaning the implementation of PortfolioValue observable
added cloneOpposite method to limit and market orders

cloneOpposite acts just like clone, with the exception that the new
order has an opposite side to the initial order. This is useful for
creating orders to close an existing position on the market.
changed the way that observables are stored by a strategy

ex: user_strategy.store( custom_label = SomeObservable(), freq = 5)
adds a custom_label field to the user_strategy instance which contains
a BufferedSeries object updated every 5 time steps with the data from
SomeObservable.

If the freq parameter is ommitted, then the BufferedSeries is updated
as often as SomeObservable.
added Candlesticks(price, volume, freq) function for use with TA-lib

Candlesticks(price, volume, freq) returns a dictionary with  ['open',
'close', 'high', 'low' [, 'volume']] time series calculated from pandas
time series containing the price and volume (optional). This dictionary
allows to easily calculate technical indicators provided by the TA-lib
library.