Move syntax check functionality to an external lib file

Move syntax check functionality (the recently merged https://github.com/gruntjs/grunt-contrib-sass/pull/147) to an external (`lib/check.js`) file in order to improve maintainability of the plugin and reduce clutter in the main task file.

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I have some questions:
1. Is there a better / more elegant way to implement this change?
2. Should I pass dependencies such as `path`, `chalk`, etc. to the `init` function to avoid duplicate `require`s in the `lib/check.js` file.
3. Should there be a comment with licensing information at the beginning of the file and should it have my name, or...?
actually none of `grunt` usage requires the instance so you could just require `grunt` instead of passing.