add getRelated operation to LiveDevProtocol.

Retrieves URLs from documents.scripts.src and document.stylesheets.href
This is an initial iteration that could eventually evolve in CDT-like
protocol to retrieve/track related docs from the browser-side.
add documentation and minor clean-up
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.
fix JSLint issues

switching from JSHint
make _scanImports to return an array of URLs

_scanImports was returning styleSheets but only href info is relevant.
@imports processing at related function was modified to handle hrefs
and to make it more readable.
fix typo and remove some empty lines
decouple DocumentObserver from LiveDevProtocol

Change DocumentObserver initialization and move it to a separated file
to make it more testable.

Restore LiveDevProtocolRemote to its initial state by removing
getRelated method and DocumentObserver injection until
we have an extension mechanism in place. DocumentObserver is now being
injected by LiveHTMLDocument as it does for RemoteFunctions.
add initial unit testing for DocumentObserver

add unittests.js file and 1 unit test for related documents.
Comments cleanup and adding .brackets.json