Make a matrix of `${TARGET_ARCH}` values

This pull request is an attempt to implement the `TODO linux 32 bit` part from `scripts/build_against_node_webkit.sh` using a slightly better approach than #322: a separate Travis CI job is dedicated to each of the target architectures.

The new release (0.10.0) of node-webkit is targeted instead of its previous RC1 version.

The x64 versions on Mac OS X currently use `false` to fail (and are added to `allowed_failures` in `.travis.yml`); it can be changed soon (the issue rogerwang/node-webkit#296 is closed, and node-webkit v0.10.1 is planned to support x64 Mac OS X).

However, `ia32` builds on Linux are failing, and I don't know why.