assigns note: add variable class, and pretty print ActiveRecord::Relation

Dunno if you're interested in this... I've done two things to the 'assigns' footnote:

1) Add the class name below the variable name, and present it a little better

2) If the assigned variable is an ActiveRecord::Relation, it'll display it as a table, rather than the raw object-as-string style.

I haven't written tests for the ActiveRecord::Relation case... I'm happy to do it, just not sure what the best approach is? Just stub everything to fake an ActiveRecord::Relation? Or is there a better way?

See before / after screenshots:

![screen shot 2014-07-06 at 10 51 41 am](https://cloud.githubusercontent.com/assets/145042/3487884/5c1a4262-04af-11e4-980f-176437625134.png)
![screen shot 2014-07-06 at 11 37 46 am](https://cloud.githubusercontent.com/assets/145042/3487885/5e010886-04af-11e4-8b21-411c10cf9f1f.png)


Signed-off-by: Joshua Paling <joshua.paling@gmail.com>