Implement signed columns for users and groups.

As discussed in #475, this adds ```uid_signed``` and ```gid_signed``` columns to ```users``` and ```groups``` tables.
Ah, I meant having the gid_signed in users when you can join gid to groups. Think it's worth duplicating the column, or is there clarity from having it in both places, when a join on the two tables is pretty performant?

Sent from my Android

> On Dec 3, 2014, at 8:15 AM, Wesley Shields <notifications@github.com> wrote:
> 
> In osquery/tables/specs/x/users.table:
> 
> > @@ -2,6 +2,8 @@ table_name("users")
> >  schema([
> >      Column("uid", BIGINT),
> >      Column("gid", BIGINT),
> > +    Column("uid_signed", BIGINT),
> > +    Column("gid_signed", BIGINT),
> Yes. At least on OS X there are some users that have a default group of -2.
> 
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