Build, test, publish Linux 32-bit binaries for node-webkit on Travis CI

This pull request implements the `TODO linux 32 bit` part from `scripts/build_against_node_webkit.sh`.

It is branched from #320 and thus depends on the same newest `node-pre-gyp` tarball (with mapbox/node-pre-gyp#95 inside) and contains the same version fix (cc0aaadce4d07bfde08d58cdb462718273e99f22) to target the recent node-webkit's 0.10.0 release instead of a former release candidate.