send-email: dependency removal, cleanup, + small feature

1 - Change from Mail::Sendmail to Net::SMTP
2 - use built-in time() instead of /bin/date '+%s'
3 - lazy-load Email::Valid and make it optional
4 - add support for mutt aliases files

Patches 1 and 3 make git-send-email easily runnable with any
reasonable Perl installation.  2 is just a good idea.  4 makes
my life a lot easier.

-- 
Eric Wong