Update the specs and add failures for some use-cases

I've updated the specs a bit to try and work out what the expectations are for this gem. 

@tomtaylor - I've included your specs

@eshear - I've included your specs


I've also added a few things: 

1. Should we be raising a warning when we try to use symbol-keys? HStore doesn't support these, so either we warn that we are converting them to strings, or we store them as ":symbol" or something so they can be converted back. 

2. What should we be doing with literals? Should {a: true} come out as {a: "true"} ? 

3. Should we allow nil keys? 


I guess this all boils down to: Should we be making sure that any hash going into hstore is always identical to the one that will come out of it? 


Please comment on the new spec file, and once we've come to some agreement I'll try and implement everything needed.