Add option for close_on_exit
Add option for close_on_exit

Added an option (close_on_exit) to only add the at_exit to close the
connection if desired. To prevent breaking existing code relying on
this, the option defaults to true (keeping existing
functionality). Setting to false will prevent the at_exit from being
added which will solve existing cases of memory leaks.

Closes #154
Fixes #156


vmware/rbvmomi,10xEngineer/rbvmomi,10xEngineer/rbvmomi