Clean up Gruntfile

* Move tasks into `grunt` directory as per [cowboy's own config](https://github.com/cowboy/wesbos/commit/5a2980a7818957cbaeedcd7552af9ce54e05e3fb)
* Add two lines to explain what happens after install
* update README
* add Grunticon
* streamline Assemble.
* other Misc fixes

ref #3