Metadata files now use .sexp extension

Since the metadata files are not actually Elisp, but just single
s-expressions.
Stopped using obsolete macro

Replaced uses of `flet' with `cl-flet'.
Added tests

ERT tests in skinny-test.el
Fixed test and hidden bug

* Fixed incorrect equality-testing function in skinny/post-meta-data.
* Added missing goto-beginning-of-buffer operation in
  skinny/post-meta-data, and moved `read' call to within the
  save-match-data.
Fixed broken macro
Fixed skinny/list-posts test

Expected value was way off.
Fixed indentation of skinny/post-meta-data
Added test for skinny/posts-html-list
Reverted to using flet (in once case)

cl-flet does not have identical behaviour to flet, so simply replacing
`flet' with `cl-flet' breaks skinny-post.
Merge pull request #4 from aidalgol/master

Followup to major changes