EchangeBase

added ExchangeBase: contains a dictionary with {asset: orderbook} pairs
PortfolioValue

added PortfolioValue: Observable that calculates an estimate of the
current portfolio value (including cash) based on the price of the last
trade.
Trader: small fixes

updates of trader's PnL and amount are now done using the property
setters.
Added new Trader class [draft in progress]
playing around with strategies
testing out a new strategy class [draft]
added try_new_trader.py [draft]