Add functions to extract X-Original-Version
Favor X-Original-Version over standard header for version info

If X-Original-Version is present, we assume that MELPA replaced the
library version with its generated package version, so we ignore the
library version and take the X-Original-Version instead.

This makes pkg-info-version-info finally useful for single-file packages
from MELPA.
[Fix #5] Merge branch 'x-original-version'

* x-original-version:
  Favor X-Original-Version over standard header for version info
  Add functions to extract X-Original-Version
Convert remaining README to ReST
Add a changelog
Release 0.5