Add custom headers support

PR for issue #8

Allows user to specify custom headers on module request, e.g. :
{
"from": "tim@wibble.com",
"to": ["bob@wobble.com", "jane@tribble.com"],
"subject": "Sadly, my aardvark George, has been arrested.",
"body": "All, I'm afraid George was found....",
"headers": [{"name": "X-My-Header","value": "Header's value"}]
}

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