Document changes to error- and function-handling
#### New features
+- Error handling at language borders has been vastly improved and is now
+ mostly automatic. Haskell's `Foreign.Lua.Exception`s are transformed
+ into Lua errors and *vice versa*. Lua-side wrappers are no longer
+ necessary.
+
+- Haskell functions are no longer pushed as userdata by
+ `pushHaskellFunction`, but as C functions. This simplifies tasks where
+ Lua expects true function objects object (for example when looking for
+ module loaders).
+
- Added stack instance for
+ Data.Set.Set,
