Kind querying in psci

I put this together in response to https://github.com/purescript/purescript/issues/527; this is my first detailed look at the code-base so I'm not 100% sure that this is what is desired. The basic idea (for now) is to follow the same logic that type querying (via `:t`) does, but check the kind of the inferences type if possible. So right now, the `:k` command only accepts expressions (and not data constructors, e.g.). It was mentioned in IRC that eventually things like `:k Either String :: * -> *` should be supported; this does not address that yet.

This being my first PR for purescript I'd love some feedback -- this was put together mostly by mockery of other parts of the codebase and I can imagine there being better ways to do what I'm doing.

Thanks!