Add jake task for generating Markdown language spec

Markdown version of language spec can now be generated using `jake generate-spec`. This task is not part of the normal build process but must be run manually after changing the Language Specification Word document. The task uses the cscript.exe script host and requires Word to be installed on the target machine.

I think you should be able to use jake.exec or our exec function in the Jakefile and then remove the import for child_process.