Enable lazy evaluation of paginate options

ref: https://github.com/amatsuda/kaminari/issues/545#issuecomment-40663427
MongoidCriteriaMethods#total_count uses :size instead of :count
Merge pull request #590 from Zhomart/master

MongoidCriteriaMethods#total_count uses :size instead of :count
Merge pull request #589 from mizoR/lazy-evaluation-opts

Enable lazy evaluation of `paginate`'s options
Do not assign options that don't have to be special-cased

`scope.total_pages` is the only method that queries count SQL so only special-case it.
Add CONTRIBUTING
Add example for friendly URLs with concern in README

Thanks to @IngusSkaistkalns for his suggestion. closes #588
Update CONTRIBUTING.md [ci skip]