Update for grunt 0.4.0, plus other fixes

I've updated grunticon-sass so that it now works with grunt 0.4.0. 

Note: **This breaks existing configurations!** Configuration properties must now be wrapped in `options: { }`. The readme file has been updated to reflect this.

As well, several configuration properties have been renamed to reflect the fact that grunticon-sass works with SCSS files, not CSS files. See the readme file.

I implemented fixes to grunticon issues [#11](https://github.com/filamentgroup/grunticon/issues/11) and [#17](https://github.com/filamentgroup/grunticon/issues/17), per filamentgroup.

I changed the task name to "grunticonsass" in the filenames, folder names, and in the code. This is because grunticon-sass is really a separate task than grunticon (and theoretically a user could use both in the same project, though I'm not sure why someone would actually do that).

I've also added a **new feature**: The user can now set cssprefix to be an empty string. Before, if the user set cssprefix as an empty string, the default value `"icon-"` would be used. (This is because an empty string is falsy when evaluated with `||`, which led to grunticonsass.js treating it as if the user hadn't set a custom cssprefix.)