add `add` helper tests
add `add` helper
add `subtract` tests
add `subtract` helper
fix `subtract` helper test
add `multiply` helper
add `multiply` helper tests
add `divide` helper
add `divide` helper tests
recompile handlebars helper with new math helpers
add documentation for `add`
add documentation for `subtract`
add documentation for `multiply`
add documentation for `divide`
Merge pull request #22 from SparkartGroupInc/math-helpers

Math helpers