improve testing in node-webkit

The following improvements are made:

* Exceptions caught in node-webkit are thrown to `stdout` using `util.log` as described in rogerwang/node-webkit#745. (The only exception is Windows where node-webkit is a GUI non-CLI application and thus `stdout` seems unavailable.)

* ` lib/util/nw_crosswalk.json` knows now that node-webkit v0.8.6 uses Node.js v0.10.22. (This is a crude temporary solution. We actually need yet another node-webkit-based application that would run on a downloaded node-webkit engine and output its `process.versions.node` to a text file or something like that. However, that matter is outside of this PR's scope.)

* `npmlog` module is used to log `stderr` of node-webkit

* an error `Cannot call method 'indexOf' of null` ([example](https://travis-ci.org/mapbox/node-sqlite3/jobs/24993716)) is prevented

* if node-webkit's `stderr` contains **only** a RANDR error such as `Xlib:  extension "RANDR" missing on display ":99.0".`, that error is ignored as [“Debugging Travis CI”](https://twolfson.com/2013-09-22-debugging-travis-ci) recommends.

Now the Travis CI build of the node-sqlite3 module can be successful again ([see a job log](https://travis-ci.org/mapbox/node-sqlite3/jobs/24996047)).

Releasing a new minor version of the npm package is recommended if and after this PR is accepted.