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 then included in `iex`'s `h` helper. 

The final result looks like this:

![screen shot 2014-12-12 at 00 19 31](https://cloud.githubusercontent.com/assets/47985/5398387/9bedc596-8194-11e4-90cb-fd9a9167f884.png)

And for functions:

![screen shot 2014-12-12 at 00 20 53](https://cloud.githubusercontent.com/assets/47985/5398423/ce253ed6-8194-11e4-9b6f-9a0e2c024dd3.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.