Add a README for CasperJS tests.

IPython/frontend/html/notebook/tests/casperjs/README.md

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+Regression tests for the web notebook. These tests depend on
+[CasperJS](http://casperjs.org/), which in turn requires
+a recent version of [PhantomJS](http://phantomjs.org/).
+
+Run the tests:
+
+```sh
+/path/to/bin/casperjs test --includes=util.js test_cases
+```
+
+The file `util.js` contains utility functions for tests,
+including a hardcoded path to a running notebook server
+(http://127.0.0.1:8888 by default).