Bumps version to 0.19a1.
warc.py: Adds attribute docstring. Changes so bytes() can be used.
recorder.py: Fixes error when cdx option is not specified.
database.py: Adds close() function. Fixes db not cleaned up.
changelog: Updates entry with note about CDX and database fixes.
changelog: Updates latest to 0.18.1.
Bumps version to 0.18.1

Merge branch 'develop'

Conflicts:
	wpull/version.py