Add 'npm install' command and retrieve sources from correct directory.

Add `npm install` command so that node module dependencies are installed before they are used (`uglify-js` in this case).

Also use a variable to store the correct directory for the sources and node_modules. This is valuable if this script (generate-combined.sh) is being called from a different directory e.g. `./some/other/path/generate-combined.sh`
Why would we run generate-combined from a directory other than where it is installed? generate-combined is tightly coupled to the files in the most recent pull. If new files are added, generate-combined will essentially be broken. I'd prefer that generate-combined works with the commit it was pulled with.