URL module integration

This expands on [MattiSG idea](https://github.com/admc/wd/pull/141) to use the [URL module](http://nodejs.org/docs/v0.10.0/api/url.html#url_url) to configure wd.

Url object, created via `url.parse` are now used to create Selenium connection instead of setting each field manually.

There are now 4 methods to initialise a browser, see README for more details:

- url string (new)
- url object (new)
- indexed params (same as before)
- named params (same as before)

This has been designed to be backward compatible.