Teach NodesHelper::excerpt to use first paragraph from body

Closes #424

Based on liamkeily's PR with modifications:
- Implement StringConverter::firstPara().  This could be further
  improved if needed.
- Kept the defaults much like the original behavior.
New convenience method NodesHelper::url()

Closes #425
Fix: NodesHelper::url() signature mismatch in php 5.4

Adapt to use `url` key populated by UrlBehavior when available

Improves #426
Fix: Upload coverage report for croogo/croogo instead of croogo/app