Bumps version to 0.20a1.
converter.py: Use copy2 instead of copyfile.
http.py: Implements handling of no content responses.
writer.py: Fixes Timestamping writer. Implements checking .orig files.
changelog: Adds entries about timestamping and http no content fixes.
changelog: Updates latest to 0.19.1
Bumps version to 0.19.1

Merge branch 'develop'

Conflicts:
	wpull/version.py