Allow custom environments

I'm currently working on an application that has an extra environment
for demonstrations. It is another Heroku instance running alongside our
production and staging environments. It shares staging behavior in that
we will occasionally copy production data over to it.

This commit makes staging a pre-defined case of a Heroku environment.
Users can make their own app binstubs for custom environments such as
"beta", "demo", etc. that can take advantage of parity conveniences.
Refactor environments and restoring

Continue to not allow Parity to accidentally modify the developer's production
database.
Bump to 0.3.0
Fix custom environment documentation

Combine "configuration" and "customization" sections.
Use consistent sh-bang lines

`bin/development` was hard-coded to use `/usr/bin/ruby`.
All the other bin files use `/usr/bin/env ruby`.
Pass arguments to `tail`

https://devcenter.heroku.com/articles/logging

If you have multiple dynos and/or dyno types
and want to filter log tailing to the process or dyno type,
you pass those filters on with the `--source` or `--ps` arguments.

This commit adds argument passthrough to the `tail` command.

Also:

* Convert single-quotes to double-quotes for affected files.
v0.3.1