Add 'importPath' option

Added the importPath compass compiler option which allows us to specify custom paths that should be searched when using the @import statement. Allows you to easily include frameworks like foundation by using: 

var app = new EmberApp({
  compassOptions: {
    importPath: [
      process.cwd() + '/vendor/foundation/scss'
    ]
  }
});
Merge pull request #4 from akmiller78/master

Add 'importPath' option