Added inventory / references file

Added a _references.md file that is an inventory of all html aliases (including path) plus an identifier and the title. This information can be used with the jekyll-references plugin (https://github.com/olov/jekyll-references) or simply as a reference file for inventory purposes etc. References allow an author to generate a link using the reference identifier (which is tightly bound to the title). This can be used, for example, in a "see also" reference scenario.