Users can mark an unread article as read via unread indicator

[samnang]

See #77

I finished the feature by letting users to be able to mark unread articles via unread indicator.

Here is what I done:

• On articles|activities index page, there is an unread indicator for each unread item
• User can click on the indicator to mark the item as read via Ajax
• Add test/integration/unread_items_test.rb for supporting feature
• Add a gem database_cleaner to support Capybara for testing javascript driver

[jordanbyron]

@samnang I took a quick look at this and this looks awesome. There are a few minor styling changes I want to make, but I should be merging this feature soon. Thanks for working on it 😄

[samnang]

@jordanbyron I'm happy to work on this, I'm learning to do some styling as well.

There are a few minor styling changes I want to make

Hope to see some feedbacks on styling as well.