extract from testium

This is the logic extracted from testium. The README.md is duplicated here for your reference.

---

# selenium-download

This module allows you to download the latest versions of the selenium standalone server and the chromedriver.

## example

```coffee
selenium = require 'selenium-download'

selenium.ensure "#{__dirname}/bin", (error) ->
  console.error error.stack if error?
  process.exit(0)
```

## api

### selenium.ensure

`ensure` ensures that the selenium.jar and chromedriver files are in the path provided.

If they are not, the latest versions of both are downloaded into that path.

### selenium.update

`update` forces the selenium.jar and chromedriver files to be the latest available versions.