Enable libvtd to be loaded by vim-vtd
Tweak vim-python interface function names

UpdatePython() was wrong; it's really the TrustedSystem we're updating.

Also, now I call EnsurePythonLoaded() every time view.vim is sourced, so python
is always available.
Add Keymap class
Put global setup at top of autoload file
Implement view-object-based API

Everything is now based on two externally-visible functions:

- vtd#view#Enter([view_type]): Enter the view buffer, creating if necessary.  If
  [view_type] is supplied, make sure the buffer has that type.

- vtd#view#Exit(): Exit and destroy the VTD view buffer.

Under the hood, I use the new s:VtdView superclass.
Consistent 2-blank-line spacing between units
Placeholder for 'Summary' and 'Contexts'

Mostly-dummy versions.  However, notice how 'j' is remapped in the 'Summary'
view, but *not* in the 'Contexts' view.
Don't setup mappings in Keymap constructor

It's more useful to be able to construct a list of keymappings in any buffer,
and later on, set all the mappings in bulk.  (This will let me add mappings
valid for *all* View types, e.g., navigational mappings.)
Add API/helper subsection headers
Add navigational keymaps for vtd view