Change what OMQuickHelpPlugin does

This version overrides the links *from* the Quick Help panel *to* the
documentation browser. Thus, Quick Help behaves normally, except that
clicking a link in the Quick Help panel invokes Dash instead of the
Xcode Organizer.
Extract code to retrieve Dash's docset settings
Invert the logic for looking up a particular platform's keyword

We figure out the platform we're looking for first, then actually go
and find it in the available docsets, instead of looping over the
docsets checking all possible platforms.
Add accessors for iOS and OSX keywords

Hide the specific details of how those keywords are determined.
Oops, platforms are found with a prefix search
Determine platform from help token's distirbution name, not the active scheme

*Much* saner all around.
Wrap all use of Xcode private APIs in a @try block
Add README.md to xcodeproj
Alter readme to describe this branch's behavior