Remove page module dependence on opening files

The site is responsible for reading files, page only operates on
buffers.
Introduce source.Filesystem

This provides an abstraction over how files are processed by Hugo.  This
allows for alternatives like CMS systems or Dropbox, etc.
Homepage "/" respects PublishDir

It wasn't taking the value of PublishDir into consideration for the
special case of the homepage "/".

Fixes #75