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. The new tag name is generated by adding one to the current build number (MAJOR.MINOR.BUILD).

#### messages.json and messages/*.txt

Files needed by Package Control to display update notifications to users.

Note: After merging this, we should add the 0.1.0 tag so that make_version.py can work.