Began implementing the new concurrency
Merge remote-tracking branch 'origin/master' into new-concurrency

Conflicts:
	core/pom.xml
	core/src/main/java/com/crawljax/core/configuration/CrawljaxConfiguration.java
Merge remote-tracking branch 'origin/master' into new-concurrency

Conflicts:
	core/src/main/java/com/crawljax/core/configuration/CrawljaxConfiguration.java
Implemented the new concurrency model in core.

- Fixed the shutdown procedure
- Created max number of states watcher
- Added a crawler context instead of passing around the session and browser
- State statistics doesnt have duplicates anymore
Fixed the last tests

Now also reporting exit status

Crawler doesnt continue when it gets lost

Working towards serialization

Fixed broken overview test

Working on screenshot failure resilience
Fixed the crawl overview plugin
Ignore mailto: links
Fixed nullpointer when body tag cannot be found
Fixed plugin executions

- Added the PreCrawlingPlugin again.

Fixes #159
Improved robustness
Merge pull request #239 from crawljax/new-concurrency-squashed

New concurrency model for core