Update core.js: Docs, Fixes

Changes:

-- Added jaws.isNumber for checking if a value passes !isNaN and
isFinite tests.

-- Changed jaws.isString to detect String-like objects

-- jaws.parseOptions now logs an unsupported property instead of
throwing an error

-- "Rect" is no longer added to the window during a call to
jaws.unpack().
(Conflicts with pre-existing window.Rect reference in some browsers.)

-- "QuadTree" is now added to window during a call to jaws.unpack()

-- jaws.log.useConsole acts as flag for if logging should use the
console or not (default false)

-- jaws.log.useLogElement acts as flag for if logging should use the
log_tag element or not (default true)