Add test drivers

These files were previously links.

Add GPL License header to source files

Add and fix test cases

Store relative heading level when parsing

Fix insertion and deletion of children elements

* recursively note_insert the children of an inserted element.
* recursively note_delete
* make mergers clone contexts before modifying them.
* make mergers indicate strategy in merge context.

Add org_heading debug module

Update README

Add gnulib and Flex generated files

This will lesson the dependencies for those who wish to build the
program.

Close input files before writing output

Properly return -1 on unresolved conflict

Add a TAP driver and parallel test harness

Add more test cases.

Disable debug printing

Implement beginning of cycle testing

- In progress solution to heading movements causing cycles in the
  document graph.

Implement parsing of properties properties drawer

Implement new doc_elt interfaces

Move out print_ctxt to it's own source

Fix issue with incorrect doc_elt_keys

Implement new isrelated and print strategies.

- Add tag merging for org_heading
- Use priorities and todo states$

Add functions to verify todos and priorities