default to sendmail command again to allow for lightweight MTAs

probably the reason why people had problems with the sendmail-command
method was because the code passed an invalid envelope-from string. Fix
that by removing the envelope from and have it set by the MTA.

Also switch from the deprecated popen2 to using corresponding methods of
the subprocess module.
Merge pull request #1 from cloph/pr_sendmail

default to sendmail command again to allow for lightweight MTAs