Add the ChecksumAccumulator companion object.

When a singleton object shares the same name with a class,
it's called that class's companion object. You must define
both the class and its companion object in the same source
file. The class is called the companion class of the singleton
object. A class and its companion object can access each other's
private members.
Move all of the scripts that won't compile out to an examples directory.

They're not really scripts, per se, just scala files that have
a mixture of legitimate-seeming code and comments (but they won't
compile!).

So, removing them from the src directory was the easiest way I could
see to make intellij happy and actually get some code to compile.
Add a simple scala application that prints the checksums of its arguments.
Use the App trait to save some typing.

As of Scala 2.9, the App trait replaces the deprecated Application
trait.

To use the trait, you first write "extends App" after the name of
your singleton object. Then instead of writing a main method, you
place the code you would have put in the main method directly
between the curly braces of the singleton object.

That's it.  Such applications compile and run just like any other.
The way this works is that the trait App declares a main method
of the appropriate signature, which your singleton object inherits,
making it usable as a Scala application. The code between the curly
braces is collected into a primary constructor of the singleton
object, and is executed when the class is initialized.