Minor fixes

1- Added documentation about Capybara server port and interface, cause people will necessarily bump into this, given the fact that Capybara by default listen only 127.0.0.1 and an mobile app won't be able to access it.

2- Added dependency for selenium-webdriver - let me know if the version suits you

3- Fixed `reset!` to avoid "NoMethodError" and `browser_initialized?` to avoid starting a new driver