Enhancements

Plugin now works fully with both CasperJS 1.0.x and CasperJS 1.1.x (tested with 1.0.4 and 1.1.0-beta3)
All integration tests run with CasperJS 1.1.x, except one which run with CasperJS 1.0.x to ensure plugin remains compatible

Here are the details of modifications :
- Refactor all integration tests to run under CasperJS 1.1.x (this will fix #15)
- Plugin will now find its files under ```src/test/casperjs``` instead of ```src/test/js```
- Plugin will now handles appropriate default values and both a light directory structure (ie ```src/test/casperjs/*.{js,coffee}``` as test scripts) and a full directory structure (```src/test/casperjs/pre.js``` as pre script, ```src/test/casperjs/post.js``` as post script, ```src/test/casperjs/includes/*.js``` as includes scripts and ```src/test/casperjs/scripts/*.{js,coffee}``` as test scripts). Of course, all the default values (for pre/post scripts, includes and scripts directories) can be overriden in the POM configuration.
- Plugin can now launch a specific test via ```-Dcasperjs.test=MyTest``` or only some via ```<includes>``` and ```<excludes>``` in the POM configuration
- All native options of CasperJS (--verbose, --log-level, --engine) are supported (fix #10 and #6)
- Plugin is now able to find the CasperJS runtime path via Maven [toolchains](http://maven.apache.org/guides/mini/guide-using-toolchains.html)
- An integration test exists for all options
- the README now presents all the options and behaviours