Syntax errors now stop the specs from running

Before it would log an error and keep on going anyway.
The Containment#protect block now throws the symbol that instructs guard
that there was an error and to stop testing.
Unload previous examples so that they do not accumulate

Fixes #30
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.