Automatically generate ChangeLog from git logs

The ChangeLog file is now automatically generated from git commit logs.
The target range of commit logs is specified using GIT_START_POINT in
Makefile.am, as the older boundary, up to HEAD. GIT_START_POINT can be
a date, a git tag, a git hash, ... See gitrevisions for more information
on specifying ranges. Also disable distcleancheck, due to the dist generated files,
to remain compatible with older Unix systems

This work is inherited from the one of Charles Lepple, on the project
NUT - Network UPS Tools <http://networkupstools.org>