Merge branch 'release/0.0.38' into develop
readme: improve instance method 'toString()' api
Merge branch 'develop' into feature/improve_instance_method_toString
src: improve instance method 'toString()'
test: add 'test/toString.js' file
readme: update instance method 'toString()' api
Merge branch 'feature/improve_instance_method_toString' into develop
readme: update instance method 'toString()' api
npm: 0.0.38 -> 0.0.39
Merge branch 'release/0.0.39'