Added type switch and casting of float64 numbers to int in Int()

[pengux]

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Fix issue #2

[owenthereal]

Do you know the type when you call it? I would expect people call something like conf.Float(key) instead of conf.Int(key)

[pengux]

Yes, the type is float64 when you unmarshal from JSON data for all numbers, both for integers and floats. I think it is correct to call conf.Int(key) if you know that it should be an integer.