Decide on what the -1 means in a strategy

[sthewissen]

Currently some of the strategies use -1 to indicate an opportunity to go short while others use it as a sell signal. It should probably be altered to act in a more uniform way. Perhaps the integers should be removed alltogether and be replaced by an enum so multiple scenarios can be supported. On the other hand the current implemented exchanges don’t support shorting anyway.

Something should probably be done though to prevent people from using a strategy in a way that it isn’t meant to be used.

[sthewissen]

This is already covered in #5 by replacing this with a TradeAdvice enum which should provide more flexibility in writing a strategy.