Add "unread-entries" attribute to toolbar button
Usage example:
	opacity: 0.85;
	filter: url("chrome://mozapps/skin/extensions/extensions.svg#greyscale");
}

Note: extensions.brief.showUnreadCounter should be true, but since this is for userChrome.css, it can be easily hidden:
	display: none !important;
}
An element with class "entry" in entry content breaks click event handler.

When walking the parent chain of the click target, we need to find the topmost element with the class "entry".
Retarget links instead of intercepting clicks.

Intercepting clicks on in-content links and opening a new tab manually
has several drawbacks. These include obtaining incorrect address if
baseURI is specified (which leads to relative links not working) and
incorrect interactions with advanced tab-management addons that do not
recognize these as child tabs (not easily fixed manually for all the
half-dozen extensions).

The approach used here is to forcibly set for every link the 'target'
attribute to '_blank'. Due to performance considerations this is done
lazily in the 'onclick' handler before the link is activated.
Fix bustage typo from commit a3726c3bad
Create README
fix async feeds cache refresh

fixes #19