Generator cleanup

* support y.g. extend inheritance method
* use [shelljs](http://github.com/arturadib/shelljs) to check Slim dependencies
* use [chalk](https://github.com/sindresorhus/chalk) for clean, better api log
* update slim generator test
* remove use of deprecated test methods

This PR also remove support for Composer and Git temporarily after use y.g. new `.extend()` method. Let generator-slim focus on Slim scaffold and another generator do they job using `.composeWith()` method.