Make most of [Reflect] getters work

Still missing the import statement.
Clean up traceur-tree script

Now that I know about the trees' built-in toJSON() methods, much less code is necessary.
Generate the import statement when using [Reflect]
Automatically lower-case the attribute name for [Reflect]

That way you don't need to do [Reflect=noShade] and such. This matches WebKit/Blink, see e.g. https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/html/HTMLHRElement.idl