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 kaminari against Ruby 2.1.1
Merge pull request #517 from seanlinsley/patch-1

Use SVG for status badges in README
Refer ActiveRecord::Base from top level namespace
Merge pull request #522 from yuroyoro/arbase_refer_from_toplevel

Refer ActiveRecord::Base from top level namespace
Use sqlite3-ruby 1.3.8, not 1.3.9

This fixes failing specs for rbx. For more details, see: https://github.com/sparklemotion/sqlite3-ruby/issues/122
platforms :ruby means MRI and Rubinius so we don't need :rbx

For more details on how to specify platform, see: http://bundler.io/v1.3/man/gemfile.5.html
Do not use test/unit for AR 4.1 tests
Do not override Mongoid::Criteria#page defined by scope

so we don't have to worry about runtime persistence. closes #326
Use the old hash syntax
rake spec:all should run spec:mongoid_40 as well
Allow failures for all mongoid 4.0 specs

It looks like #with option doesn't work right now. Since mongoid 4.0 is not
released yet, just allow them to fail for now. We'll come back later.
Do not include railties and rspec-rails gem [ci skip]

They are unneeded if Sinatra is used.
Test Kaminari::ActionViewExtension only when Rails is defined
Revert "Use SVG for status badges in README"

This reverts commit d1be2150f098abdb261b0cd752499c582b1f4262 because the badge
for travis doesn't show up correctly.
Exclude basic form params from pagination links

Even though the basic form params will be blacklisted, other params
submitted from a form still gets appended to the links. However, there
is no good way to detect what should be excluded and what shouldn't. If
a particular parameter(e.g. params[:user]) should be excluded, the
example below could be a workaround:

    paginate @users, params: {user: nil}

fixes #131. fixes #182
Use NextPage/PrevPage class in link_to_next_page/link_to_prev_page

So these methods will generate consistent urls with `paginate` method.
similar impl to #424 but no fix for `#rel_next_prev_link_tags`.

Note that this change will introduce the behaivor explained in 2fd7d36b72af73d2506f8e2ab68704d804f70fc5.

fixes #124.
Do not use unicode characters

Ruby 1.9.3 doesn't like them.
Add docs badge to README
Merge pull request #529 from rrrene/patch-1

Add docs badge to README