feat: support coverage per spec

Coverage reporter now listens after each spec end for coverage objects. If such is found it is merged using istanbul collector. At the browser end, the merged object is written into the report file.
Added handleling of no coverage on run complete - this is necessary,
because e2e scenario adapter sends the coverage objects on each spec
complete and does not send anything on run complete.

lib/reporter.js

@@ -67,6 +67,8 @@ var CoverageReporter = function(rootConfig, emitter, helper, logger) {
     var collector = collectors[browser.id];
     if (cov && collector) {
       collector.add(cov);
+    }
+    if (collector) {
       pendingFileWritings++;
       helper.mkdirIfNotExists(path.resolve(outDir), function() {
         var now = dateformat(new Date(), 'yyyymmdd_HHMMss');
@@ -81,6 +83,12 @@ var CoverageReporter = function(rootConfig, emitter, helper, logger) {
     }
   };
 
+  this.onSpecComplete = function(browser, result) {
+    if (result.coverage) {
+      collectors[browser.id].add(result.coverage);
+    }
+  };
+
   this.onRunComplete = function(browsers, results) {
     browsers.forEach(function(browser) {
       var collector = collectors[browser.id];