Add omitted bitmap require statements and prototype constructor.

Heyo, this was really easy to fix because I just looked at what was done in other files. This is important for when there's a fair number of maxChoices, and is easy to miss because of that branch in code; if your code always has 1 for maxChoices, this works great.

Also, I notice that the "Adding a buffered attribute" shorthand that's available in the Ruby implementation is not available. Will you be able to add that? You can find a little documentation on that feature here:
https://github.com/NYTimes/tamper/tree/master/encoders/ruby#adding-a-buffered-attribute