Rename source files.

This is the first step toward modularizing the library per #1332. It resolves #1881.

– The source file names are now slugs
– The source files no longer have the `marionette.` prefix
– Unified the style of the header comments in each source file
– Updated the build files to reference the new source files
– Updated the spec runner to reference the new source files
– Updated the node unit test runner to reference the new source files