Add support for a verbose flag

Setting VERBOSE=1 in the environment will now make the buildpack run npm with verbose flag.
Buildpack can rerun in verbose for helping debug

If the configuration variable BUILPACK_RETRY_VERBOSE is set, the buildpack will rerun npm again in verbose mode so that you can enjoy nice concise logs when all goes well, and precise logs when you need inspection.
Merge pull request #7 from mackwic/master

Add conditional verbosity helping debugging