If measuring time across blocking calls, such as thread synchronization, one currently must time.time().  This is because time.clock() measures cpu seconds on unix.  On windows, however, time.clock() would be more appropriate because it measures wall-clock time.

To avoid having to put platform clauses everywhere, this patch adds time.wallclock().
The current implementation is a simple alias to time.clock on windows and time.time otherwise.  Future improvements may add a better implementation on those non-windows platforms that support it.