Add task to TODO list.
Add dynamic features to country table.

Changes:
* Add visit checkboxes to country table's rows with immediate AJAX action.
 * As relation with collection item established through currency and collector
   never visit a country without collecting a currency I able to not to show
   cuntries without currencies. So, I hide such countries.
 * Add ability to respond to JSON requests to CountriesController#update.
* Add Select All checkbox with dynamic selection of all table rows.
* Add Send button to apply result of selection made with "Select All" checkbox.
* Add "Search" field to filter countries by name and code.
* Fix action which send ajax request with visit state to act only on the rows
  with different checkbox and status states, i.e. rows selected/unselected with
  "Select All" checkbox.
* Update pie chart as countries visited/unvisited
* Add visual indication on row updated with AJAX.
* Fix Cucumber tests.
Move task from TODO to CHANGELOG.
Add country list screenshot to doc/ directory.