Quick cleanup of proposed libDRAMPower api.

I attempted to minimize exposure of the guts of the power model. I'm not totally happy with exposing the "counters" class-instance to the outside world, since it is prone to change later. It may be better to opt for a similar structure as used for the Energy and Power.
Made all cycle counters int64_t.

Also reduced the status of most functions in CommandAnalysis.cc from public to private, and made 3 variables local to a function instead of them being a member variable.
Changed name of timings variable back to counters.
Made command list public.

This allows the library user to sort the list before the power calculations.
Implcitly this means the memcommand class is now (temporaraly?) part of the API.
Merge pull request #19 from ravenrd/lib_api_proposal

Lib api proposal