implement node-webkit-based module validation

Fixes #47.

Contains fix #49 entirely (i.e. is based on its branch) and thus may fix #46 unless #49 is applied earlier.

**Important note:** `stderr` of node-webkit had to be ignored because it's always non-empty (`Xlib` complains about `RANDR` there). See “[Debugging Travis CI](https://twolfson.com/2013-09-22-debugging-travis-ci)” for details.

In order to demonstrate the patch I have a [Travis CI job log](https://travis-ci.org/Mithgol/node-sqlite3/jobs/20798716) where node-pre-gyp successfully performs node-webkit-based validation of the `node-sqlite3` module.