add box score to gather line score data

[earlonrails]

I wanted to gather line scores since the dataset I was using had errors in their line score data. This PR adds that functionality to pybaseball.

from pybaseball import boxes

team = 'DET'
date = '2010-07-19'

boxes(team, date)
             team      line_score runs hits errors
1   Texas Rangers  20300010000002    8   19      3
2  Detroit Tigers  11102001000000    6   13      2