respect meteor package namespacing

hey, this is a very simple PR to make sure marked doesn't attach to window/global and instead works through meteor's namespacing.  that means, that, it will be available (always) via `Package['chuangbo:marked'].marked` and won't interfere with window/global when `{weak: true}` is used.  it also means the package will work with famono :), which currently breaks the package because of how `define` is used.