Change types of Mouse Signals to Float.

This was suggested by Brian Slesinsky on the mailing list https://groups.google.com/forum/#!topic/elm-discuss/6iejCR164WU

```
I'd recommend using float everywhere for coordinates, even things like Mouse.x that are currently Int. Even if it's an integer now, browsers could switch to float and it wouldn't even be a type change as far as JavaScript is concerned. (For GWT, our Element API's use ints because that's what browsers did at the time, and migrating every function signature to float is painful. Better to fix it when the language is young.)
```

This also avoids conversion from int -> float. 
He has some more useful suggestions, I think.