Fix issue with visual tab at end of line

Making a visual selection that went to the end of line and invoking
snipmateVisual caused the cursor to move one column left of where it
should've.
Clean up plugin/* a little
Add support for buffer local scope aliases
Add SnipMateLoadScopes command

This is like the UltiSnipsLoad command that came out of
<https://github.com/tpope/vim-rails/issues/145>
Fix handling of single stop snippets

After expanding, the snippet would jump to the zeroth tabstop but still
be active.
Update docs

Document :SnipMateLoadScopes and buffer-local scope aliases