Better management creating clients

This update change you connect and send email. Useful for create multiples clients and send with persistent connection with a lot of connections

-NewSMTPServer() change to NewSMTPClient() and return new SMTPClient struct
-New() change to NewMSG()
-Public function Send() only receive SMTPClient struct
-Private function send() receive SMTPClient struct instead SendTimeout, Client and KeepAlive
-Double verification in send, first that SMTPClient struct is not nil and Client is not nil
-Public function Connect() return SMTPClient struct
-Private function checkKeepAlive() only receive SMTPClient struct
-Updated readme

Note: this update is for v2.1.0 and is not compatible with v2.0.0