Added email reminders

Fixes issue https://github.com/chicagopython/chipy.org/issues/50

It looks like your using heroku so add the Heroku Scheduler add-on by running.
```heroku addons:add scheduler:standard```

Then open it
```heroku addons:open scheduler```


Add the following jobs:

This queues the emails.
```python manage.py send_reminders``` - run once a day

This sends them in the queue.
```python manage.py send_mail``` - run once a day a few hours after the messages are queued


NOTE: when the commands are run I believe a new dyno is spun up until it is finished.

Right now the emails are in this format
```
Hi {rsvp name},

There's a ChiPy meeting this {day of meeting}. 

Location:
{venue name}
{venue phone} / {venue email}
{venue address}

Topics:
{topic name} by {presenter name}
{description of topic}
```