Added basic mailer module and email for new members

Closes #44.

Reasons this shouldn't be merged:
- No tests for `mailer.py` - I cant think of a good way to test this, any suggestions?
- Haven't tested the mailgun mailer as I don't have an API key
- `mailer.py` should be decoupled from `mhvdb2.app` so that it can be called externally (e.g. from a cron job for renewal emails)
- My email for new members is probably bad and needs rewording