Fix minor TimingFunction validation issue

This fixes a rather uncommon issue in TimingFunction validation. Steps:
1. Try to open an Inline Editor on `animation-timing-function: cubic-bezier(0, 0, 0, 0,)` (notice the trailing comma)

Result: Console error, no inline editor
Expected: Inline editor should auto-correct the invalid function
@MarcelGerber This is an interesting case. Due to the [regex used to parse the cubic-bezier string](https://github.com/adobe/brackets/blob/master/src/extensions/default/InlineTimingFunctionEditor/TimingFunctionUtils.js#L41), the last value is "everything past the 3rd comma and past whitespace". So there could be a lot of weird stuff in here -- too much I think to try to parse with our own regex.

So, I looked at the [code that currently traps the exception](https://github.com/adobe/brackets/blob/master/src/extensions/default/InlineTimingFunctionEditor/BezierCurveEditor.js#L66) and it's using the unary operator to convert from string to number. I think we still need `parseFloat` to make sure it's a base 10 number, but this code seems to be simpler, solve the problem, and all unit tests still work:

```js
        if (typeof str !== "string") {
            return { isNumber: false, value: null };
        }

        var val = parseFloat(+str, 10),
            isNum = (typeof val === "number") && !isNaN(val) &&
                    (val !== Infinity) && (val !== -Infinity);
```

What do you think of this solution?