add install script to build pdfjs src on package install

@jonmccracken-wf 
;) I really don't want to add another tool to the stack. So I dug more into the npm thing and we can do this so that we get the pdf.js hydra built on install.

This will run "node make generic" within pdf.js hydra after it is pulled down as a module.

alternatively, we can within pdf_client_service add something like this to the pdf_client_service package.json:
    "build" : "npm run-script pdfjs test",
then we can just do this to get a rebuild of pdf.js:
`npm run build`

or manually do the pdf.js re-make like this:
`npm run pdfjs test`  (with the current)
`npm run pdfjs build` (if we change it from "test" to "build")
`npm run pdfjs install` (if we change it from "test" to "install" like below)