Add Windows support

You can host the binaries elsewhere if you'd prefer. You don't have access to a 32-bit Linux machine by any chance, do you? It looks like there are some precision issues in the tests for win32:
```
julia> Pkg.test("Dierckx")
INFO: Testing Dierckx
ERROR: assertion failed: |get_residual(spl) - 0.0| <= 1.7105694144590052e-45
  get_residual(spl) = 1.3349971792853637e-33
  0.0 = 0.0
  difference = 1.3349971792853637e-33 > 1.7105694144590052e-45
while loading C:\Users\Tony\.julia\Dierckx\test\runtests.jl, in expression starting on line 20
```

I found the `@unix?` macro in `src/Dierckx.jl` was really picky about being on a single line, so keep an eye out for that.