initial monitoring of added/removed related docs

based on MutationEvents.
initial migration to MutationObserver interface
remove getRelated call from initial sequence

it now entirely relies on events coming from the browser.
all messages are enqueue and sent once transport is connected.
fixed some issues:
- isRelated implementation
- delete related stylesheets
add DocumentObserver as watcher for related docs

- code re-organization
- fix initial sequence (initial event with all related docs + watching)
- remove msg queue (related docs observation starts now after document
  is already loaded)
- adds handler for related docs in LiveHTMLDocument
change method name from getRelated to Related

There was an inconsistency in previous commit btw
DocumentObserver and protocol.
Merge pull request #5 from sebaslv/related-docs

initial monitoring of added/removed related docs