0.6dev: Implement a `subprocess` implementation of `execute()` that is now the default for all slaves where this is available - essentially all slaves with Python 2.4 and higher, or where module is installed separately. This fixes: * #256 - correct interleaving of `stderr` and `stdout` on Windows * #257 - missing timeout on Windows * #425 - `popen2` is deprecated * #346 - trapping execution errors (where process cannot be spawned at all) * #311 - trapping argument errors (graceful message when arguments don't match) * #262 - add a 1 second delay if a step finish in same second as previous step * #226 - cleaned up log messages, and remove some ambiguity that could arise * #101 - better logging for os errors * old Windows `execute()` forgot to reset `cwd`

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