ENH: slight performance + to Win QPC timer

- Made a small change to the ctypes Win QPC timer implementation that
saves ~100 usec per call.
- Changed MonotonicTimer class so that the initial start time value can
be passed in as a param to the class __init__. (This is needed by the
ioHub process, as it gets told by the experiment process what the global
timer initial offset should be).
- Clarified timer test file comments at start of file, correctly what
tests were actually implemented.
Merge pull request #289 from isolver/master

small changes to Timer related ENH.