Implement check

[ntindall]

Fixes #35 #82

npm install --production
npm-shrinkwrap check

this should detect both additions and deletions to the package.json file that are not reflected in the shrinkwrap, as well as inconsistencies with node_modules/

@Raynos, could you take a look?

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[ntindall]

@Raynos @Matt-Esch ping on this.