Refactor #rel_next_prev_link_tags method

closes #501
Fix tests for Sinatra
Tweak spec for sinatra again
Merge pull request #500 from aptx4869/support-mongoid-max-scan

Add support for mongoid max_scan option
Add link_to_previous_page helper for sinatra
Merge pull request #504 from ikeay/fix_sinatra_previous_link

Add link_to_previous_page helper for sinatra
Add tests for mongoid 4 beta 1
Merge pull request #505 from arthurnn/mongoid4

Add tests for mongoid 4 beta 1
Do not test mongoid 4.0 with Ruby 1.8.7
Mongoid 4.0 with rubinius is still unstable so just allow it to fail

Also, AR edge with jruby has been stable for a while, so do not allow it
to fail.
Add documenttion for link_to_previous_page [ci skip]
Make helper methods for sinatra slightly better
Another small refactoring
Test padrino-helpers 0.12.0
Fixes #452
Merge pull request #511 from zzak/padrino_test

Test padrino-helpers 0.12.0
Update README.rdoc

clarify that paginate_array does not work on the array inline, but instead returns a new PaginatableArray.
Use instance variable in paginate_array example from #515

https://github.com/amatsuda/kaminari/pull/515#issuecomment-35795767
Use SVG for status badges in README

http://shields.io/ provides many standardized badges for OSS. The PNG versions provided by Shields are similar to the images currently used, but the SVG versions added here are more legible on high pixel density displays.
Test activerecord ~> 4.0.0 in active_record_40 gemfile

This changes from using '>=' which would install the latest version of
activerecord available at the time of running `bundle install`.
Support testing activerecord 4.1 from git.

This could be changed once Rails 4.1 is released to use rubygems, but it
might also be interesting to test against the stable branch for
regressions.