Backend changes. Beginning of scripting support.

DataManager:
 - Added "feedsMatchingTag" function, which returns all feeds which have the given tag (or all feeds, if tag is nil or empty string), with option to allow incomplete tags.
 - Added a bunch of calls for scripting. None of them are tested, though they're also not hooked up.
 - Created background javascript virtual machine and a background context to go with it, that way I don't have to generate a new context and such each refresh of query feeds.

Removes old (unused) content subview when a new article is pushed or pulled on the ArticleViewController article stack.

Made the loading bar in FindFeedViewController the dark green used throughout the app.

Added "id" fields in the Feed and Article asDict() methods.