'Crumb trail' navigation

- Extend the 'select last directory' functionality to work for multiple
  directories, think $OLDPWD vs $DIRSTACK.

- Fix - and <Enter> to work for remote files/directories too.
  (Used :Explore, :Rexplore and the original - and <Enter> mappings
  instead of :edit %:h, to work along with netrw, not instead of it.)
Saner alternate file switching

<C-^> in a netrw window will switch to the last open _file_ for *that
window*, if there was one. It will never switch between two netrw
buffers.

The dirstack will be flushed regardless of the way you switched from
netrw to a file - <C-^>, :b, :e, etc. (In the last commit it only worked
properly with <CR>)
Revert an accidental change
Preserve # when descending too

Forgot to guard descent with <CR> against changes to the alternate file
name. Now <C-^> goes back to the last open file buffer, regardless of
how many times one may have navigated up and down the directory tree.
Ditch (broken) remote browsing

I just broke it with the last commit.
Fix filenames with spaces
Merge pull request #1 from gkirilov/crumb-trail

'Breadcrumbs' navigation and sane alternate file switching