Refactor background script into modules

[sliminality]

Currently, only pdiff functionality is separated into its own class. Should probably refactor BrowserEndpoint into domains. Maybe something like this:

• BrowserEndpoint manages connections and interfaces directly with debugger
  • getActiveTab
  • initConnections
  • onRequest
  • debugEventDispatch
  • sendDebugCommand
  • cleanup
  • onSocketDisconnect
  • onDebuggerDetach
• BrowserAction anything related to the browser action
  • updateIcon
  • TODO: context menu stuff
• DOM anything to do with querying the DOM
  • getDocumentRoot
  • selectNode
  • highlightNode
  • addParentIds
  • getNodeId
  • getNode
  • searchDocument
  • getOffsetParentId
• Styles reading and writing styles
  • getStyles
  • refreshStyles
  • toggleStyleAndRefresh
  • toggleStyle
  • isDisabled
  • isDeclaredProperty
  • resolveProp
  • propExists
  • getScreenshotForProperty