Refactoring 'functions' method.
Delete a TODO and add new TODO.
Tweak.
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
Update README
Refactoring operator
Implement Symbol#super, #sub method
Implement log2 function and its tests.
Implement log10 function and its tests.
Merge pull request #15 from huyu398/dev_log2/10

log 関数の挙動
Fix integrand_spec for improving integral calc
Improve integral calc
Merge pull request #24 from gogotanaka/improve_integral_calc

Improve integral calc
0.1.31
Update README
Improve diff spec