ID: 51690573 Add calculate feature
Modify a feature that calculate billing data.

Add test case.
Consider a discount in the calculation.
Change the data format.
Modify method name
Fix a bug that range specification is wrong
Remove inappropriate stub method and run the test by preparing the actual data
Modify to the private method and remove the test case
Modify descriptions of spec and add comments.
Move files to subdirectory
Merge remote-tracking branch 'origin/master'