show scopes even if no results are found

When using filters, the previous behavior was to hide all scopes. This
is counterintuitive, as a different scope from the current one may have
results for the selected filter, and ends up confusing the user.
add conditional :if and :unless logic to filters

+ followed the way conditional procs are built for Scopes
+ by way of Array.group_by, we now only recognize the last-defined
  `filter` call for each individual attribute
+ update the specs to allow `filter` to be called multiple times on
  the same pseudo-document
routing fixed for Devise on root namespace
allow custom CSS class for rows in attributes_table
Fix fields_for_params when Boolean values
Add the license identifier to the gemspec
remove duplication in ResourceDSL#*_action methods
simplify options normalization helper
refactor router
Improved Czech translation
Allow resource collections to be sorted by virtual columns
Index page title can be set using a proc
Update documentation for how column sorting works
update docs for custom pages

+ added `page_action`
+ general rewrite of file
Merge pull request #1851 from Daxter/update-pages-docs

update docs for Custom Pages
Merge pull request #1841 from romansklenar/patch-1

Improved Czech translation
Merge pull request #1831 from goshakkk/refactor

Some refactoring
Merge pull request #1827 from nbibler/master

Add the license identifier to the gemspec
Add cucumber scenario for sorting by virtual columns
Updated to set the page layout when rendering custom actions (same as ResourceController).  Ugly cut and paste, but moving determine_active_admin_layout to BaseController seemed uglier without bigger changes.
(cherry-picked from 9eb53e1)