Collections plugin: add index to global metadata, bug fixes

- add index_limit argument
- pattern is optional now, and defaults to all files
- files that do not contain the metadata used as sort key
  all last, in arbitrary order
- fixed a bug for empty collections
- add full metadata of the files to the index
  and the previous and next pointers
Templates: do not prefix global metadata with 'site.'

This plays better with whisker, because that does this kind
of prefixing automatically for composite metadata fields.
Branch plugin, a'la metalsmith.
Replace and replace_meta plugins
Change .path to _path

Dots are not playign well with whisker templates.