Adds the ability to provide a single url.

This provides:
- Better error handling.
- Ability to provide a single URL.

Steps to test:
- Checkout this branch.
- Adds a '<property file="build.properties" override="true" />' to the build.xml (we don't want this committed).
- Create a build.properties file with:

```
browserstack.base=http://previousnext.com.au
browserstack.tunnel=true
browserstack.user=<username>
browserstack.pass=<key>
```

- For a single url run:

```
phing browserstack -Dbrowserstack.singleUrl=http://previousnext.com.au
```