Refactor helper
Add Function class
Add specs for function_spec.rb
Initialize Integrand
Separate Delta class
Improve somethings
Add examples to function.spec
Implement multi function
Improve #differentiate
Add example to function_spec
Add #-@ method
Assert Float cal
Improve integral and add spec
Improve specs
Merge pull request #9 from gogotanaka/integral

積分を実装する