Merge pull request #7 from thijsc/master

Add SIsMember command
Added header
New contributor (from #7)
Removed authors

They're in the AUTHORS file already.
Adding SRandMember

Smells like we should have a ErrInvalidQuery in this case, so 'count'
has to be always bigger than 0
Adding SetNX . following same int/bool shenanigans
Adding SREM