Remove main stack trace from default reporter (closes caolan#23)

I've come to the understanding that mot of the stack trace are
irrelevant, thus this commit will remove all of it except the file and
line number of the second entry in the stack trace which empirically
matches the probable invocation in the test code

lib/reporters/default.js

@@ -22,6 +22,11 @@ var nodeunit = require('../nodeunit'),
 
 exports.info = "Default tests reporter";
 
+Error.prepareStackTrace = function(error, structuredStackTrace) {
+	// our invocation will be at the second level of the stack trace
+	var our = structuredStackTrace[1];
+	return "\nat " + our.getFileName() + " line " + our.getLineNumber();
+}
 
 /**
  * Run all tests within each module, reporting the results to the command-line.