Ignore concerns in app/models/concerns

Fixes #1636
Add ability to inherit custom fields from RailsAdmin::Config::Fields::Association

field.properties instance variable is lost when you configure new custom field
Merge pull request #1736 from substantial/remove-new-object-id

no reason to explicitly assign an id to a new object
issue 1689: fix menu highlighting with browser back/forward buttons

Pjax only runs inline JS once, so the model highlight on the left nav
didn't change when users navigated with the back and forward browser
buttons.  Added some code in ui.coffee to run code inside script tags
when pjax notifies of a "popstate" event.
fixed capitalization of dashboard title
js localization for filters
Add json field datatype
Fix build error caused by BSON namespace change in Mongoid 4.0. Closes #1764
Fix improper use of raise_error matcher
Merge pull request #1756 from mixan946/translate_filter_values

js localization for filters
@bindings was not preserved on proxying. Fixes #1777
Use find_in_batches ActiveRecord feature to deal with large record sets in export.
Point mongoid-grid_fs back to original repo
Associated models with a has_one relation is not created by default, it's now up to the user to explicitly create it.
Minor refactoring.
Fixed spec for nested form. Had to activate testing through Webkit to make it work.
Edited Travis config to run specs with xvfb.
Replaced capybara-webkit with poltergeist to get better support for Travis. It was not without cons though, had to set some values in the spec with jquery since poltergeist gave me all sorts of trouble.
Removed config.use_transactional_fixtures = false since it wasn't necessary
Changed DatabaseCleaner settings not to mess with mongoid.