Remove plugin artifacts
Gitignore ./plugins
Added instructions for adding the plugin to the project
Fixes #89
Include addition of the plugins subdirectory to the FileViewerPlugin
instructions.
Provide clear feedback on attempts to add public share rooms that are
already being visited.

Plus, indicate that the share room is already present, but adjust the
"remember" attribute if the re-add changes it.
Added showNotify() to DialogView to do transient feedback.
Icomoon warning icon added
Move all user interaction - share room already present and share room not
found - out of the share rooms collection and into the share rooms view.

The already-intricate provision for toggling share room remembering for
already present ones would get even more intricate, so just drop it.
Use new spiderOakApp.dialogView.showNotify() to show a transient dialog
from AddShareRoomView.form_submitHandler().