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.
We should keep our private interface minimal since we are mixing it into AR

```ruby
def sneaky_save
  sneaky_save!
rescue ActiveRecord::StatementInvalid
  false
end

# Saves the record raising an exception if it fails.
# @raise [ActiveRecord::StatementInvalid] if save failed.
# @return [true] if save was successful.
def sneaky_save!
    new_record? ? sneaky_create : sneaky_update
end
```