added option to specify engines for executing tests, fixed casper integr...

I hope you find this pull request useful! I added a new "engines" option to the options field in the casperjs grunt task. This lets you specify either PhantomJS, SlimerJS, or both as targets. Leaving this option out will still execute the tests using PhantomJS as they did before, so it should be backwards compatible. I did add the engines option to the plugin's Gruntfile.js just to show the option, it isn't necessary, but I thought it was useful to include.

I also fixed the integration tests, they were still using options from the previous version of casperjs and would not work correctly. You should be able to run grunt from the command line now and see the test execute.

Let me know if anything doesn't look right in this PR and I can update it.