Simplified the calculation of total portfolio value in Portfolio

[nme01]

The value of self.equity is incremented by the position's P&L. In my opinion there is no need to calculate unrealised P&L once again, since there is already a field for that in the Position class. The field is updated on each Position.update_market_value() call.

[coveralls]

Coverage remained the same at 57.143% when pulling 8265881d6b5b15c4c27ef1fd670151b8d06c9a13 on nme01:master into dfa66932a882f38c5d172ee3d5e28871806f3ad1 on mhallsmoore:master.