Fixes bugs in functions _round and validate that caused bugs in lab-rgb ...

Had some bug in LAB to RGB conversion.
Example : 
```js
lab = {
                l : 12.319070000421839,
a: 32.79724522241495,
b: 19.351723549763015
              }
    rgb = Color.convert(lab, 'rgb');
```
The problem being:
* _round sometimes has to round values in exponential form. The regex doesn't consider that. So we could use toFixed first and parse via regex.
* A bug in validate function - empty check with '!' which considered 0 as empty (invalid) and assigned minimum value for it. Updated the check considering that.