Hierarchical collapse and hide input extensions

Hierarchical collapse now support move,delete,undelete of collapsed cells (i.e. multi-cell operations).

Both hierarchical_collapse and hide_input moved are moved into their own subdirectory. In these directories one can also find filters/preprocessors/templates for nbconvert that make sure one gets the expected behaviour when building slides. At some point this functionality should be moved into IPython, because at the moment a new template file is needed for every document type nbconvert can generate. This can be circumvented by adding the hide_input/hierarchical_collapse functionality into one of the base template structures.