add a method to get group suggestions for a profile

GET
http://api.linkedin.com/v1/people/~/suggestions/groups:(id,name,is-open-
to-non-members)
LinkedIn::Mash#all returns [] instead of nil when there are no results
Merge pull request #199 from hundredwatt/all_mash_method

Change LinkedIn::Mash#all to return [] instead of nil when there are no results
Merge pull request #198 from kjpatel/feature/group_suggestions

add a method to get group suggestions for a profile
Warning about post_group_discussion