switch to FSC to determine Printf specifiers

@dungpa FCS reports specifiers for the following:

```fsharp
let _ = sprintf "%.7f %7.1A %7.f %--8.1f"
```

![image](https://cloud.githubusercontent.com/assets/873919/5171236/887677da-7427-11e4-918c-9bd80d59e251.png)

It's test https://github.com/vasily-kirichenko/FSharpVSPowerTools/blob/new-printf-specifiers-colorization/tests/FSharpVSPowerTools.Core.Tests/SourceCodeClassifierTests.fs#L1614-L1618

```
Expected: equivalent to < <(2, [(Function, 8, 15)])> >
But was:  < <(2, [(Function, 8, 15); (Printf, 17, 21); (Printf, 22, 27)])> >
```

I think it's a bug in FCS.