Views: view callbacks and custom view handling

[kellyredding]

This overhauls the view setup for assert. There are two big changes here:

• views are now rendered using callbacks fired by the runner.
• documentation and helpers for using assert with with custom 3rd party views

[kellyredding]

@jcredding - look over it and weigh in on pros/cons. I'm going to break up assert-view and see how nicely this works in practice. Anyway.

[jcredding]

@kellyredding - one quick note, are you using rdoc or markdown for your README? It's a .rdoc file, but this assert view section looks like it's supposed to be markdown? You can see it not formatting some stuff here:

https://github.com/kellyredding/assert/tree/view

[kellyredding]

@jcredding - Its using rdoc right now, but I have become so accustomed to writing markdown that I keep adding it in. I'll have a follow up pull request where I convert the readme to markdown.

(thanks for the detailed review, btw)

[kellyredding]

Rebased and re-pushed the commits. Ready to merge. @jcredding - any problems with a me merging this in?