Auto-reloading when files change

[n6g7]

For faster development (no need to rebuild image when changing files).

Ideas:

• create new HTTPDF_DEBUG env var. following behaviour only applies when set to true (defaults to false)
• watch $HTTPDF_DOCUMENTS_SRC for changes
• on change:
  • recompile files (using babel)
  • re-run resolver.buildIndex()
  • print some sort of message to let user know files were reloaded
  • ...
  • profit