WiX tasks

A pull request for issue #51.

Usage (Candle):
```
WiXCandle("./Sources/*.wxs", new CandleSettings {
    Architecture = Architecture.X64,
    Defines = new Dictionary<string, string>
    {
        { "Foo", "Bar" }
    },
    Extensions = new [] { "WixUIExtension" },
    OutputDirectory = "./obj/"
});
```