Refactoring Client.getGroupMembership().
Refactoring Client.getGroupMembership().

Adding jsdocs, simplifying code.

wakashige/stormpath-sdk-node,stormpath/stormpath-sdk-node,stormpath/stormpath-sdk-node,peebles/stormpath-sdk-node,ck-lee/stormpath-sdk-node,peebles/stormpath-sdk-node