Environment variable to determine number of worker processes

I ran into a few occasions where I wanted to spin up more worker processes on my development machine. I liked the simplicity of `unicorn-rails` but I really wanted some way of doing lightweight configuration.

I've been on an "environment as configuration" kick thanks to Heroku so I figured I'd implement worker count that way.

### Usage

Spins up 2 worker processes.

```bash
$ UNICORN_WORKERS=2 rails server
```

Still works.

```bash
$ rails server
```