Fix #16: Make npm work with sbt subprojects,

where package.json is in the subproject directory.

Also added a scripted test using this functionality.

https://github.com/sbt/sbt-js-engine/issues/16

Thanks a lot to Thomas Feng <tfeng@linkedin.com>
for the original fix.
Merge pull request #19 from kolloch/npm-fix

Make NPM work in sub modules