smtp.class.inc: provide access on the smtp protocol

This is a class that provide access on the smtp protocol to send one or more messages with them.

version 0.7.1-dev:
- support for socket extension and stream functions
- IPv6 support (But this I had never tested)
- New Trac/SVN page for better support on future request and bugs
http://trac.assembla.com/smtp-class-inc
- using of php5 exception handling mechanism
- multiple message sending support over one connection (The old class doesn't support this)
- planned cram-md5 authorization