Consolidate the code to approach a first release

[wrznr]

A number of fixes for issues resulting from test runs:

• Correctly Place structures which are not on top of a page
  • Retrieve ALTO files via a dedicated struct link member of the class Mets
  • Move text retrieval to Alto class
• Set corresp and facs attributes of pb elements
  • Store links to DEFAULT images in METS
  • Store ORDER atrribute of filePtr

Add a Changelog

Fixes #28