Heinous speed issues with ALEAgentHost

[DaveyBiggers]

Rough timing tests (on my sluggish VM):
ALEAgentHost.sendCommand takes roughly 10-15ms.
Of that:
ALEInterface::act is taking about 0.3ms
The frame scaling/copying is taking (on average) around 0.3ms

The remaining 95% of the time appears to be spent in the reward code - largely, I suspect, in serialising/deserialising the reward XML message.

Needless to say, it should be possible to speed this up.

[DaveyBiggers]

This constructor is the culprit:
TimestampedReward::TimestampedReward(boost::posix_time::ptime timestamp,std::string xml_string)
It is taking around 10ms.