Handle spaces in path to node and path to external r.js

Changes to `NodeJsRunner` so that:

* node may be in a directory with spaces in the path
* an external require.js (the "optimizer file") may be in a directory with spaces in the path

Also includes:

* a new `NodeJsRunnerTest` unit test, which uses a couple of shell scripts to substitute for the real node.js
* updates to OptimizerTest that uncomment and add tests verifying spaces in the paths are OK

These changes might fix issue #8.