Abort check-examples target if phantomjs takes too long to run examples.

Currently a WMS server (http://demo.opengeo.org/geoserver) seems to be down, which results in a neverending `integration-test` target:

    2013-03-12 09:13:12,232 check-examples: phantomjs bin/check-example.js build/gh-pages/unselectable-css-const/examples/wms-single-image.html?mode=raw

We should probably abort `phantomjs` after a certain amount of time has passed.