Add SendGrid API support

[Akkarinage]

Second attempt, because yesterday I failed at git.

Adds the following new configuration. If this is not null, all mail sending within FluxCP will go through the SendGrid API instead.

'SendGridAPIKey' => null,	// SendGrid API Key for sending mail. (https://sendgrid.com/docs/Classroom/Send/How_Emails_Are_Sent/api_keys.html)
				// If this API Key is set, it will be used instead of the SMTP settings.

Example:

'SendGridAPIKey' => 'SG.myap1k3y',

I'm considering adding a few other mail APIs so we can drop phpmailer once and for all.

[Akkarinage]

Has anyone tested this yet? I get no issues using this but I can't speak for all of us.

[HopeItBuilds]

aren't there other open-source/free alternatives to phpmailer other than this paid one (free 100 mails/day for 60 days is not even acceptable) to be added to a open-source/free tool like fluxcp?