fixed test for Travis-CI
simplified usage
some changes

- appended npm-script
- changed Travis-CI script
- launch Firefox and PhantomJS in Travis-CI
fixed doc comment
appended @private
appended null check
appended unit test of getObjectKeys_
appended fallback function of Object#keys
linted
update ltsv.min.js
appended update description
changed version to 0.7.1
release for version 0.7.1