[WIP] Convert to Module

#### Problem

Previously you had to create an instance of **Alephant::Scout::Url** to check if an URL was valid, even though the class was not holding any state. 

```ruby
Alephant::Scout::Url.new.valid? 'http://bbc.co.uk/news'
```

#### Solution

Convert it to a module, and update the unit tests.

```ruby
Alephant::Scout::Url.valid? 'http://bbc.co.uk/news'
```

#### Other

- [x] Refactored the unit tests.
- [x] Add missing dependencies.
- [x] Start using [rake-rspec](https://github.com/revett/rake-rspec).
- [x] Update README.
- [ ] Add integration tests.
- [ ] Use [Faraday](https://github.com/lostisland/faraday) instead of [HTTParty](https://github.com/jnunemaker/httparty).
- [ ] Add Travis CI job.