fay and fay-base

yesod-fay depends on fay, and also on fay-base implicitly, so it should be seamless to add these.

fay 0.16.0.2 is updated to support haskell-src-exts 1.14, so either add a constraint for `fay < 0.16.0.2` or wait for #103 to be resolved, I guess?

I was also wondering, what's your stand on adding fay specific packages to stackage?