Adds #sneaky_save!

* This is similar to `ActiveRecord::Base#save!`. Our use-cases always need a whiny `sneaky_save!` (similar to when you would prefer to use `save!`) as we never expect it to fail. When it does, we want it to complain loudly with the inner exception. Instead of repeating that pattern in our code-base, we've pushed it into this gem.

* The specs had to be re-org'ed a bit to accommodate its testing.

@einzige Let me know what you think.
```ruby
to raise_error(YourErrorClass)
```

your code may fail with any random error (eg NoMethodError).