Add missing values support

Support null values in data. This can be used for forecasting:

EG:
```
			// Data
			var data = [

				[-11, null],
				[-10, -738],
				[-9, -520],
				[-8, -350],
				[-7, -222],
				[-6, -130],
				[-5, -68],
				[-4, -30],
				[-3, -10],
				[-2, -2],
				[-1, 0],
				[0, 2],
				[1, 10],
				[2, 30],
				[3, 68],
				[4, 130],
				[5, 222],
				[6, 350],
				[7, 520],
				[8, 738],
				[9, 1010],
				[10, 1342],
				[11, null],
				[12, null],
				[13, null],
				[14, null],
				[15, null],
				[16, null]
			];
```

Regression JS will fill the null following the chosen regression.
Merge pull request #8 from amaury-d/master

Add missing values support