Auto install Casper.JS when the module is pulled from the NPM repository

It would make people's lives a lot easier if they didn't need to worry about also installing Phantom.JS or Casper.JS and just be able to add the grunt-casperjs NPM module and have NPM take care of the rest.

This patch solves this. It will use the existing NPM phantomjs module and set the PHANTOMJS_EXECUTABLE env property when kicking of casperjs command line.
In addition it will download the casperjs 1.0.2 zip from github, unzip it locally and the use the binary in this unzipped directory when kicking off casperjs.