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
Fix fields_for_params when Boolean values
Allow resource collections to be sorted by virtual columns
Update documentation for how column sorting works
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)
Added new scenario to to ensure custom actions render the active admin layout.
(cherry-picked from 5eb853e)
Refactored #determine_active_admin_layout into the BaseController
Merge pull request #1849 from azach/master

Allow resource collections to be sorted by virtual columns
Merge pull request #1819 from getcomparisons/patches/fields_for_params_false

Fix fields_for_params when Boolean values #1818
Merge pull request #1809 from Daxter/fix-devise-routing

Fix routing for Devise on root namespace
Merge pull request #1804 from Daxter/559-show-scopes-if-no-result

[559] Show scopes if no results are found
Merge pull request #1801 from Daxter/938-conditional-filters

[938] Conditional logic for filters