implement build/job cancelling
document cancel api
Merge branch 'master' into rkh-cancel-build

Conflicts:
	travis.gemspec
Merge branch 'master' into rkh-cancel-build

Conflicts:
	travis.gemspec
Merge branch 'rkh-cancel-build' of https://github.com/travis-ci/travis into rkh-cancel-build
regenerate gemspec
Merge branch 'master' into rkh-cancel-build

Conflicts:
	README.md
	travis.gemspec
Merge pull request #72 from travis-ci/rkh-cancel-build

implement build/job cancelling