Modify build_ctags for ctag option support

- Alter parameters taken by build_ctags, so tag generation can be limited to
  a single file, all files in a directory or all files in a directory plus
  subdirectories
- Document altered build_ctags and resort_ctags functions
- Move unit test code from ctags.py to test_ctags.py
- Define new unit tests for modified ctags file
- Move README on ctags from ctags.py to CTAGS.rst file. Alter formatting