Added rw_company_admin permission

This perimission is missing. It's documented here https://developer.linkedin.com/documents/authentication

Author: lucavandro

linkedin/linkedin.py

@@ -15,6 +15,7 @@
 __all__ = ['LinkedInAuthentication', 'LinkedInApplication', 'PERMISSIONS']
 
 PERMISSIONS = enum('Permission',
+                        COMPANY_ADMIN='rw_company_admin',
                         BASIC_PROFILE='r_basicprofile',
                         FULL_PROFILE='r_fullprofile',
                         EMAIL_ADDRESS='r_emailaddress',