text: Title: Unnecessary requirement of selenium with `from splinter import Browser`
  Body: When using `from splinter import Browser` to only use e.g. `Browser("django")`, it fails without selenium being installed due to: https://github.com//cobrateam/splinter/blob/986ce0a10c52f08196b32b91f752182cb7517892/splinter/browser.py#L14
  
  This is only used within https://github.com//cobrateam/splinter/blob/986ce0a10c52f08196b32b91f752182cb7517892/splinter/browser.py#L73-L85, and could be handled through some conditional import, i.e. without selenium being installed/importable, this exception would not happen - and more specifically is only expected to happen for a certain set of the browsers (`splinter.driver.webdriver.*`).
  
  It would be great if the wrapper could be made to not require installation of selenium for non-webdriver-based clients.