v1.2.0: and fix version number in manual

See https://groups.google.com/d/topic/ninja-build/m3Ftf0iBLI8/discussion for the v1.2.0 release notes.

v1.1.0

See https://groups.google.com/d/topic/ninja-build/YiOyUMTppfk/discussion for the v1.1.0 release notes.

v1.0.0

See https://groups.google.com/d/topic/ninja-build/oeJiFkmbQ4g/discussion for the v1.0.0 release notes.

release-120715

See https://groups.google.com/d/topic/ninja-build/aDHpOVmVEg4/discussion for the release notes for this tag.

release-120508

See https://groups.google.com/d/topic/ninja-build/wtfGmg0IoXQ/discussion for the release notes for this tag.