fixed incorrect hashrate on mac os and fixed some memleaks

[gjwang]

[DavidVorick]

Thanks for the pull request!

We'll get to reviewing this as soon as we can.

[lukechampine]

Looks good overall. I would prefer to see the device info printed in a nicer format. Aside from that, just make sure you follow our current style, specifically:

• Tabs for indentation
• Open brace on the same line, separated by a space (e.g. foo() {)

[lukechampine]

Excellent. I think this is good to merge, but I would like to confirm that it behaves as intended on Windows and Linux as well. @DavidVorick, can we push this out to some miners and get their feedback?