Add composer support

[VolCh]

This simplify use smtp-validate-email in composer controlled projects even without publishing on https://packagist.org/

[zytzagoo]

Thanks, this looks great!

Gonna tweak a few tiny details after merging write them down here so I dont forget :)

• ever since https://github.com/zytzagoo/smtp-validate-email/commit/fc4cc5a7f844feea3ed7e4c00a6d953b9361de53 and https://github.com/zytzagoo/smtp-validate-email/issues/25 the minimal php req should really be 5.4 I think
• gonna change source/docs links probably
• see what to bring over from https://github.com/zytzagoo/smtp-validate-email/pull/18 and close that one

Thanks a lot!