Created an API to stop Crawljax from a plugin or from the Runner.

Closes #270
Updated the changelog
Merge branch 'master' into feature/270-provide-api-to-stop-crawljax

Conflicts:
	CHANGELOG.md
Merge pull request #274 from crawljax/feature/270-provide-api-to-stop-crawljax

Feature/270 provide api to stop crawljax
Merge branch 'master' into clean-up

Conflicts:
	core/src/test/java/com/crawljax/core/CrawlerTest.java