Nodemailer.sendMail should be purely asynchronous.
Merge pull request #285 from jsdevel/forcing-async-in-sendMail

Nodemailer.sendMail should be purely asynchronous.
Make the Date header RFC compliant.

While debugging some email using
[this message linter](http://tools.ietf.org/tools/msglint/), I came across the
following warning:

`WARNING: old-style timezone in date in header 'Date' at line 1`

This led me to invstigate further and I then stumbled across
[this](http://cr.yp.to/immhf/date.html):

```
There are two possibilities for the time zone:

* Numeric. An atom containing a sign and 4 digits. +hhmm means +(hh * 60 + mm) minutes, and -hhmm means -(hh * 60 + mm) minutes; mm is always between 00 and 59.
* Obsolete. One or two atoms, each starting with a letter. 822 specified the nicknames UT and GMT for +0000, EDT for -0400, EST and CDT for -0500, CST and MDT for -0600, MST and PDT for -0700, and PST for -0800. (822 also specified military nicknames for some zones, but it got them backwards; handling them would be pointless.) Many messages use other nicknames, violating 822. See below for a list of possibilities.
```