Support paging when fetching organization members

Organization members are a paged list in the V3 GitHub API. Previously the code would fetch only the first page of ~30 members and return that. This change switches the return to a PagedIterable<GHUser> so clients can fetch the entire organization's member list.
add a filter param to the get members method
Add org public-members call, to complement the full members list

http://developer.github.com/v3/orgs/members/#public-members-list
Fix org.getMembers() to 'wrap' returned users for post-populating

This fixes an issue introduced with PR #66 in commit fee02a3d16 - the
`getMembers()` method switched to using paging, but lost the code that
populated the users with a full set of values.

This commit just applies 'wrapping' to all users returned, so that methods
like `user.getCreatedAt()` can be called on returned users without throwing
a NullPointerException - the reference to 'root' allows the user objects to
populate themselves.
Keep the original signature intact for backward compatibility
Merge branch 'pull-76'