Change composing way, compose as function composition

Ref #3
Move composable spec to tube spec
Remove accept? from Tube
Update README
Add sink composable spec
Fix Composable :bug:
Remove accept? from Sink
Fix for API changes
Add todo
Add more specs fro sink composability
Add associative law for tube
Merge pull request #4 from taiki45/change-composing-way

Change composing way, compose as function composition