text: Title: In Sample Program - Instantiation of Browser class should also include the browser name parameter
  Body: **_What is the Issue ?**_
  
  In https://splinter.readthedocs.io/en/latest/tutorial.html. When creating an browser instance we need to pass it a parameter, i.e the name of the browser. But the ReadMe doesn't state anything about it. We have this issue when the issue when the use has more than one browser installed.
  
  **_The Error that we see if no parameter is passed during Browser instantiation is:**_
  
  ```
  Traceback (most recent call last):
    File "./sample.py", line 6, in <module>
      browser = Browser()
    File "/usr/local/lib/python2.7/site-packages/splinter/browser.py", line 63, in Browser
      return driver(*args, **kwargs)
    File "/usr/local/lib/python2.7/site-packages/splinter/driver/webdriver/firefox.py", line 39, in __init__
      self.driver = Firefox(firefox_profile)
    File "/usr/local/lib/python2.7/site-packages/selenium/webdriver/firefox/webdriver.py", line 81, in __init__
      self.binary, timeout)
    File "/usr/local/lib/python2.7/site-packages/selenium/webdriver/firefox/extension_connection.py", line 51, in __init__
      self.binary.launch_browser(self.profile, timeout=timeout)
    File "/usr/local/lib/python2.7/site-packages/selenium/webdriver/firefox/firefox_binary.py", line 67, in launch_browser
      ......
  ```
  
  **_How to resolve this:**_
  
  It would be better to clearly state in the line no. 3 of the sample program ( https://splinter.readthedocs.io/en/latest/tutorial.html ) that the user might need to pass the name of the browser as a parameter, something like this:
  
  `browser = Browser('chrome')`  or 
  `browser = Browser() # you might need to pass parameter(i.e your browser name) during Browser instance creation Browser('name of browser')`
  
  I am reporting this because I recently started using the Splinter API and I found it hard to figure out why the sample program code was not running. 
  
  Hope this helps someone else as well 😄 
  Thank you!
  