expose file path relative to source dir to liquid

Fixes #633.
fix path for posts: must include "_posts"
add cucumber tests for page.path variable
Move Jekyll::DEFAULTS to Jekyll::Configuration::DEFAULTS
Add Jekyll::Configuration
Jekyll.configuration should use the new Jekyll::Configuration class
TomDoc Jekyll::Configuration
Unit test for Configuration#stringify_keys
Add unit tests for Configuration#config_files
In Configuration#config_files, cascade to the Configuration's value of 'source' before the default
Refactor setup block for test of Configuration#config_files
Added a unit test for Configuration#backwards_compatibilize
Moving the backwards-compatibilizing to Configuration#read_config_files. @mattr-
Merge pull request #951 from maul-esel/liquid-file-path

Expose file path to Liquid
Update history to reflect merge of #951
Add new pagination variables to the Variables page in the docs site. #942
Add note in docs about page.path. #951.
Refactored Configuration#config_files
allow overriding page.path in YAML frontmatter
add cucumber tests for overriding path