Add script to generate changelog

scripts/make_version.py

	* will update CHANGELOG.txt by adding all the summary
	  lines from all commits since the last tag,
	* will generate messages/<NEW_TAG>.txt message file based
	  on the previous tag's,
	* will update messages.json to point to
	  messages/<NEW_TAG>.txt

Note: No tags are created by this script; the new tag has to be
added manually.

messages.json and messages/*.txt

are files needed by Package Control to display update
notifications to users.
Merge pull request #113 from guillermooo/scripts

Add script to generate changelog