Add 46elks sms, which has phone number saving built in

This adds support for 46elks, which allows sending SMSes.  But it also has built in support for tracking phone numbers for users.

```
Hubot> hubot shell has phone number 36632325
Hubot> Ok, shell has phone 36632325.
Hubot> hubot give me the phone number to shell
Hubot> Shell has phone number 36632325.
Hubot> hubot shell has phone number 36632325
Hubot> I know.
```

Source script: [46elks.coffee](https://github.com/github/hubot-scripts/blob/master/src/scripts/46elks.coffee)
Tagging @doomspork / @jmacadam for review