refactoring things related to tweet dataframe including changing class name

This refactoring allows me to understand code more clear and assign it's
responsibility more clear. Also I intended to the class holds it's data.
After I learned about Software Engineering from this semester, I can
consider relationship between data of class and responsibility of it.
I will keep going to refactor this class again.
Merge branch 'master' of https://github.com/sangheestyle/bison