fix differential of sin
Refactoring 'functions' method.
Delete a TODO and add new TODO.
Tweak.
Improve function_spec
Add distributive for formula
Use -@ method
Improve helper.rb
Refactoring formula class
Refactoring  lib/dydx/algebra/inverse.rb
Remove Num#==
Merge pull request #17 from hiromiso/fix_sin

fix differential of sin
Update gemspec
0.1.29
Merge pull request #18 from huyu398/refactoring

関数メソッドのリファクタリング
0.1.3