suppresses jshint `Use '===' to compare with 'undefined'` warning
adds String#slugify()
export a variable to global to avoid duplicate requires
update README
add String#isString()
cache toString
add Boolean#isBoolean()
add String#isString() to doc
Merge branch 'master' into develop

Conflicts:
	README.md
update readme
add Travis CI
add String#toCamelCase()
Merge branch 'develop'