Merge pull request #67 from geraintluff/gh-pages-merge

Website overhaul
Fix broken link
Fix broken link in documentation.html
Latest versions of core and validation specifications
Updated link to hyper-schema spec
Add tv4, and place numbers next to sections
Note down two known implementations supporting v4.

Move the "Java" section up - there is only one validator, but it's v4-compliant, so we probably want it to be easy to find.