add string_to_integer function to prairielearn.py
add string_to_integer function to prairielearn.py

mwest1066/PrairieLearn,mwest1066/PrairieLearn,parasgithub/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,parasgithub/PrairieLearn,mwest1066/PrairieLearn,tbretl/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,tbretl/PrairieLearn,tbretl/PrairieLearn,rbessick5/PrairieLearn,rbessick5/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn,parasgithub/PrairieLearn