Adjusting ExecutableNode docs to make things a bit clearer.

Also adjusted line formatting to suit my ocd...
Adjusting Despatcher docs to make things a bit clearer.

Also adjusted line formatting to suit my ocd...
Tidying some functions signatures
Renamed ExecuteUI to DespatcherUI
Renaming despatcherParameters to despatcher.
Moved requirements metadata to the new ExecutableNodeUI
DespatcherUI sets metadata for ExecutableNodes only.
Moving ExecuteButton creation to ExecutableNodeUI.

The definition of the button remains in DespatcherUI.
Removing ExecutableRenderUI

This was just being used to hide the requirement plug from the graph, which we now want visible. We may well add the file back later on, but I didn't want to leave an empty file around.
Unifying exposed nodules for the various ExecutableNodes.

Fixes #109.
Re-implemented DespatcherUI.ExecuteButton as a plug value widget.

Also made it private, moved it's registration from ExecutableNodeUI to DespatcherUI, and added some comment sectioning in DespatcherUI.
Updated nodeGraph startup script to use DespatcherUI
Merge pull request #867 from andrewkaufman/executableNodeUI

ExecutableNode UI improvements