Add poltergeist and capybara-webkit adapters

This is a major refactoring that supports for selenium, capybara-webkit, and poltergeist capybara drivers. It's inspired by #9, this allows additional flexibility and a decoupling of components.

EDIT:
One of the reasons I decided not to support #9 was because when this gem would be downloaded they'd have to download and install selenium and capybara-webkit. This refactoring assumes that the environment will provide the appropriate libraries. This refactoring also makes it [super easy](https://github.com/vrish88/capybara-accessible/commit/f7a18aa14ac287140891ea26375b771a0822269a) to add another adapter.

Thoughts?