Feature: Add a 'pull_db' command

This command will (for now) either pull from Heroku or from a local SQL file.

\### Pulling From Heroku

If you would like to pull from the production environment and you're using
Heroku, all you have to do is:

```
pull_db -a 'app_name'
```

\#### Pulling a Specific Backup

By default this will create a new Postgres backup file and pull it down.  That
way you know you have the latest version on your system.  Sometimes however, you
want to pull down a specific backup.  Maybe it was one that was taken prior to
a deploy.  In order to do this, just specify the `-b` option like so:

```
pull_db -a 'app_name' -b 'b123'
```

_Note:_ to get a list of your backups simply run `heroku pgbackups`

\#### On Specifying App Names

By default you want to only specify the _base_ app name.  In other words, we
expect that your app is named something like:

```
app_name-environment
```

on Heroku.  So for the `-a` option, you would only specify `app_name`.  The
command will take your app name and combine it with the environment to create
the full Heroku app name.

\### Pulling From a Local File

Typically your workflow goes something like this:

* Pull from production
* Try some stuff (hackity hack hack) which messes up the DB
* Pull from production again
* Hack some more

Fortunately, if you've previously done a pull, the local raw SQL file is still
on your system in the `tmp` directory.  Each file you've pulled is kept with the
timestamp as the filename.

If you would like to reuse that file (and you should), do this:

```
pull_db -a 'app_name' -e 'local'
```

This will pull from the SQL dump file with the latest modified time in the tmp
directory.

\#### Pulling From a Specific Local File

If the latest file is not what you want, you can alternatively pull from
a specific SQL dump file by specifying the path.  Here's an example:

```
pull_db -a 'app_name' -e 'local' -b './tmp/my-specific-backup-file.sql'`
```

--------------------------------------------------------------------------------
Change-Id:     Id59e534aac9604f2cdf6e3112cdf83fdc100040b
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>