Ginga v2.6.3

• Fix for issue that stops ginga startup when loading externally distributed plugins that have errors
• Fix for an issue loading plugins from the command line when they are nested in a package
• Added bindings for moving +/- pixel delta in X or Y and centering on the pixel
• Fixes for some key mappings for tk, matplotlib and HTML5 canvas backends
• Fixes for IRAF plugin under python 3
• Fix for a bug using remote control (RC) plugin from python2 client to python 3 ginga
• Documentation updates