Added functionality for selecting the scope of a token

Added the functionality for selecting the scope of the token.

Example: 

client = LinkedIn::Client.new(LNKD_API[:api_key],LNKD_API[:api_secret])
request_token = client.request_token({:oauth_callback => "URL",
                                          :scope => 'r_fullprofile+r_emailaddress'})