Prototype for evaluation on the server.

Not integrated in the server yet, but you can use it on it's own:

```haskell
session <- startSession "."
evalInSession "1 + 2" session
--= Right "3\n"
evalInSession "putStrLn" session
--= Left "\n<interactive>:6:1:\n    No instance for (Show (String -> IO ()))\n      arising from a use of `print'\n    Possible fix:\n      add an instance declaration for (Show (String -> IO ()))\n    In a stmt of an interactive GHCi command: print it\n"
endSession session
```

Comments, suggestions?