Class and example reloading

This PR contains a few different changes. Please review carefully.

### Use Rails 3.2+ class reloading when available

Additional custom_reloaders can also be passed in via options to support
other frameworks like Rails 2.x or custom class reloaders.

I wrote support in `Reloaders` for prepending, but I didn't hook it up to jruby-rspec. Probably before this is merged, we should decide to either drop prepend support or support a way to prepend reloaders. We might also consider supporting a way to replace the default reloaders. I wanted to get your opinion on this whole `Reloaders` thing before I fleshed it all out.

This closes #29

### Unload previous examples so that they do not accumulate

This fixes #30

### Syntax errors now stop the specs from running

Before it would log an error and keep on going anyway, sometimes scrolling an important error off of the screen. The Containment#protect block now throws the symbol that instructs guard that there was an error and to stop testing.