Avoid unnecessary node-gyp clean on install

Building this module takes forever, so we really don't want to clean
unless it's necessary. Avoiding `node-gyp rebuild` that is defaulted to
the preinstall script and instead invoking the configure and build phases
explicitly. Adding a 'clean' script to this package that will invoke
`node-gyp clean`
Merge pull request #6 from Vannevartech/rebuild

Avoid unnecessary node-gyp clean on install