Preparing for 1.5.1 release.
rename configure.in to configure.ac to be liked by autoconf 1.12

aclocal-1.12: warning: autoconf input should be named 'configure.ac', not 'configure.in'
Add GPL3 copyright headers
Bump version to 1.5.2
"Removed ChangeLog"
Add empty ChangeLog to allow build from git
Bump version to 1.6.0
Merge branch 'master' of git://github.com/mate-desktop/mate-common