amberc.js: assign AmberC instance to module.exports

it is now possible to use require('amberc') to
retrieve an instance of the amber compiler

bin/amberc.js

@@ -739,5 +739,7 @@ AmberC.prototype.closure_compile = function(sourceFile, minifiedFile, callback)
 	);
 };
 
+module.exports = new AmberC();
+
 var amberc = new AmberC();
 amberc.main();