Fixed freeze and added selection functionality

I removed the deprecated method DocumentManager.setCurrentDocument(doc) which caused Brackets to freeze and added functionality to retrieve a page directly from a user-selected URL.
When no text is selected the dialog appears.
A protocol is now added to the URL when none is specified.

I also implemented the $.ajax call with the Promise methods of the jqXHR object and added a fail() callback which simply logs that the specified URL could not be opened.