Improvement to ISSUE #102. Revamped CrawlQueue.java to make it completely thread-safe in addition to many details mentioned in the comments in the file. CrawlerExecuter.java is modified to make the thread pool size fixed and use the bounded version of CrawlQueue. CrawlerExecutorTest is modified to address ISSUE# 75 and to test further for the revamped CrawlQueue and CrawlerExecutor.