Replace PanelManager for WorkspaceManager

To update the extension with new API of Brackets and to avoid the
message log  "Use WorkspaceManager.createBottomPanel() instead of
PanelManager.createBottomPanel()."
Merge remote-tracking branch 'upstream/master'

Conflicts:
	main.js
Remove PanelManager
Merge pull request #18 from dalcib/master

Replace PanelManager for WorkspaceManager