Add NpmInstall post-checkout, post-commit, post-merge, and post-rewrite hooks

[jawshooah]

Planning on adding similar hooks for package managers like bundler and bower. Might be worthwhile to add a base InstallDependencies module since the code will be pretty much identical.

[sds]

Merged in 3b41109 and 6778db2 (squashed so that the build doesn't break if we were to git bisect).

Thanks!