Add compileDependencies option

Redux of last week. This is the same operations, but runs faster and is moved to an outside resource.

I have added the option `compileDepedencies`, when true this adds a partial's dependent files to the current async process. I have added sass-get-dependencies as a dependency on the package.json. Written tests and updated the documentation / Readme file.
In sass-get-dependencies there is a check to make sure you aren't returning repeated files.

```
if (curStatement.test(contents) && dependentFiles.indexOf(file) === -1) {
	dependentFiles.push(file);
}
```

You're correct that if you send this lots of partial files currently you will recompile the same sheet multiple times. This can been adjust with a quick check to the this.files array though.