More daemon option information

@jpetazzo @unclejack @alexlarsson @crosbymichael 

Can you please give us some other juicy tradeoffs between the storage drivers? 

I'm presuming that the statement I made about the lxc driver isn't too wrong, but that too needs review.

@jamtur01 @fredlf @ostezer - at this point, we have zero documentation helping users/admins decide which to use when - I _hope_ this is an improvement.
exec?