Support for short form hex colors for StyleSheets

The current implementation allows StyleSheet color properties to be set with hex strings or html color strings, for example

```
borderColor: '#d6d7da',
color: 'red'
```

However, short form hex colors do not work:

```
borderColor: '#ccc', // throws 'InvalidColorString'
```

Short form for hex colors should be supported for similarity to CSS.
