Loading from a file works

Completes #6 - the ability to execute scripts from files. For every full expression, if the eval'd expression results in a number, the number is printed; otherwise, nothing is printed. For example,
```
(defun square (x) (^ x 2))
(square 9)
(square 4)
```
will print
```
$ python calc.py test.calc
81
16
```