Added follow_company and unfollow company

Added follow company and unfollow company options as detailed in the API [here](http://developer.linkedin.com/documents/company-follow-and-suggestions) I integrated the changes gemmaros@f3caeb7afc211c57055ebf7b976efc94c58b464f made in their PR as I needed them for the gem myself, hope that's not an issue. added tests and all tests are passing. Let me know what you think. Thanks for the gem, it's been helpful.