Renaming existing 'Group' into 'BlockGroup'
Implementing Group fragments
Be sure Document#get return null if not found
Fixes for PHP 5.3

- replace `[]` by `Array()`
- avoid `$var->meth()[$i]` (use `$t = $var->meth(); $t[$i]`)
Merge branch 'group_fragments'
Now using GitHub Releases instead of changelog.md