Fix DirectionFactory to support shorthand syntax without unit (Fixes #17)

[aminroosta]

@jacobp100 This small change will support margin: 10 or padding: 2 5.
This Fixes #17

[aminroosta]

The last unit test

it('throws useful errors', () => {
  expect(() => transformCss([['margin', '10']]))
    .toThrow('Failed to parse declaration "margin: 10"');
});

Is not an error at all, margin: 10 is perfectly valid 😢
So i changed it to margin: #fff which should actually give an error.

[jacobp100]

We've made the decision to require units where the CSS spec requires units. Thanks anyway!

[aminroosta]

@jacobp100 Thanks!
A more descriptive error message like Failed to parse declaration 'padding: 5', CSS spec requires units may help.
I'm not the first one to open an issue/pr for this :-)

[alvelig]

What units are required for react-native?

[johnpaulada]

@alvelig I tried dp, and it didn't work so I used px and it worked.