Add company search

A new method has been added to the API::Companies module. The method, company_search, takes in a keyword(s) + any additional filters/options and makes a request to the LinkedIn company_search endpoint. 

Several tests have also been written to ensure this method can handle different combinations of arguments and still return the desired data. I added the oauth_token onto the target uri in each vcr cassette test (for consistency with other cassette tests for the gem).  
@maxkohl88 What's with all of the blank lines at the end?