Suggested fix for OPEN-45.  I'd like to decouple Executable as much as possible from assumptions and state of autobuild.  To that end I added an optional argument to the __str__ method to accept an options list to insert into the command string.  The command string generated by __call__ and __str__ is now identical.  Incidentally I made the environment an optional argument to __call__ to make it more general and decouple it a little more from the rest of autobuild.