Start Mink logic onCommited instead of once the page is fully loaded

[machawk1]

The webNavigation API provides a hook to identify when a user had committed to a page, i.e., when loading begins and part of the initial resource has been received. This would be a better hook to start the Mink logic, since the URI is also committed and used as the basis for query to the Memento Aggregator. The current implementation waits document.ready.

[machawk1]

chrome.webNavigation.onCommitted.addListener fires fine from mink.js, a background script, but the listener in content.js, a content script, never catches the message. The displayUIBasedOnContext() function reside in the content script, so message passing must be used.