refactoring

[kevinmichaelchen]

• using money.proto
• using big.Float for converting Money to/from float64 (unit-tested in money_test.go)
• unit test for scoring function

Generated here with:

\text{trip_score} = 
100
+ 
\underbrace{(w_c * c)}_{
  \substack{
    \text{expected} \\ \text{payment}
  }
}
+
\underbrace{(w_p * p)}_{
  \substack{
    \text{time to} \\ \text{get there}
  }
}
+
\underbrace{(w_t * t)}_{
  \substack{
    \text{expected} \\ \text{trip time}
  }
}