Add type and spec information to help function

This is just quickly hacked together, but should be enough for initial feedback. Today I played around with types and typespecs a bit, and thought it'd be nice to have them included in `iex`'s `h` helper. 

The final result looks like this:

![screen shot 2014-12-12 at 00 26 25](https://cloud.githubusercontent.com/assets/47985/5398506/8cf47502-8195-11e4-9117-7d3f03a40160.png)

And for functions:

![screen shot 2014-12-12 at 00 25 13](https://cloud.githubusercontent.com/assets/47985/5398491/65777312-8195-11e4-9df8-dbc11d45b648.png)

Before I spend any more time on this, I wanted to clarify a couple of things:

1. Does anyone besides me care for this functionality?
2. Is the current implementation sensible?

If the answer to the above questions is "yes", I can polish this off a bit, add specs etc. I'm also open to suggestions on how to change/improve this.