Add -x and -s parameters to uploadexec

Added -x parameter to the script which indicates that the underlying
meterpreter session should be terminated when the execution has
finished.

Added -s parameter which takes a floating point number as an arg
which indicates the number of seconds to sleep between uploading
and executing. This helps in the case where http(s) payloads are
used for meterpreter and a time delay is needed to make sure that
the file has been written to disk and the lock released prior to
attempting to executing it.
Use Rex.sleep instead of sleep
Land #2673, -x and -s for uploadexec meterp script