Implement SendRawEmail

[camjackson]

Resolves #5.

Changes:

• Implements the SendRawEmail function, including base64 decoding of the raw message
• Responds with an error to unsupported actions. Previously it would just never respond, leaving the client to eventually time out
• Wrote unit tests for new code
• Upgraded node version on travis from 5.6 to 5.10, in order to use Buffer.from
• Chose the sushi emoji 🍣 to represent 'raw' email requests 😁
• Tested it manually and it works!

Notes for reviewer:
The diff, when viewed all at once, looks a bit scary because:

• The first commit extracts a bunch of code into a separate module
• The second commit contains a big de-dentation which makes it look like I changed a bunch of lines when really it's just whitespace

So to make the review easier, I recommend:

• Reviewing one commit at a time rather than all at once; and
• Put ?w=1 onto the end of the github URL, which makes it hide whitespace changes so you can see what actually changed

Happy to make changes wherever needed 🙂

[coveralls]

Coverage increased (+4.03%) to 92.126% when pulling be6a4a7 on camjackson:sendrawemail into 9c91a41 on Si1kIfY:master.

[camjackson]

@Si1kIfY any chance you could take a look at this? I've been using it myself and it works well.

[csi-lk]

Merged and released, thanks mate

[camjackson]

No worries, thanks for merging it!