Frontmatter defaults

This PR implements general defaults for YAML frontmatter variables, as @benbalter suggested in [#969](https://github.com/mojombo/jekyll/pull/969#issuecomment-20344309). The defaults are specified in `_config.yml` and can be restricted by type (post, page or draft) and path.

## DONE
- [x] basic implementation
- [x] fix for Ruby 1.8.7
- [x] more robust cucumber features
- [x] more robust path matching
- [x] inline code documentation
- [x] documentation for jekyll.rb
- [x] document precedence handling

@mattr-, @parkr what do you think?