First crack at an "update" command.  Lots to do still.
Use set_head() instead of calling update() directly.
Add convenience methods that delegate to the head.

get_distribution() should be moved too, but I'm going
to postpone that for now.
Tinkering with "update" command.
Trailing spaces.
Mention "update" command in the change log.
Edit POD.
Fix critical bug.

Must take care when mutating hash refs.
Just whitespace.
More work on fixing sticky wicket.

The fake index records for core modules will now only
include non-deprecated ones, since Pinto and cpanm both
treat those as if they are already removed from perl.
That way, Pinto will fall back to an upstream repository
for those modules.  And hopefully, one of them will
actually provide it.