MJSON objects are instantiated when the file is loaded, not required

[cdebost]

JS files are processed in two steps: first the file's text is put into memory on load, and then the contents are actually executed when the file is required. MJSON files, however, are both loaded and executed on load.

This causes issues in mop, which has to load modules but does not want to execute them, as there may be some runtime-only logic in object constructors that could crash the build. Objects should be instantiated on require instead.

A temporary work around for mop is in montagejs/mop#94