Visual Studio support

[jpvantuyl]

I'm able to get to the following in VS2015 on Windows:

> study_koans()
The very basics : 123456789abcdefghijkl

1. Failure (at ../koans/on_calculations.R#6): Using R as a calculator ----------
1 + 1 not equal to _
Modes: numeric, NULL
Lengths: 1, 0
target is numeric, current is NULL