Added new IAM functionality

Added Go APIs for:
 * GetGroup
 * GetGroupPolicy
 * GetInstanceProfile
 * GetRole
 * GetRolePolicy
 * AccountAliases
 * GroupPolicies
 * GroupsForUser
 * InstanceProfiles
 * InstanceProfilesForRole
 * RolePolicies
 * Roles
 * UserPolicies
 * Users