Rule proposal: jsx-curly-spacing

This is similar to [object-curly-spacing](https://github.com/eslint/eslint/blob/master/docs/rules/object-curly-spacing.md).

There are two main options for the rule:
- `"always"` enforces a space inside of curly braces

``` .js
<Component
  foo={ prop1 }
  bar={ prop2 }
/>
```
- `"never"` disallows spaces inside of curly braces

``` .js
<Component
  foo={prop1}
  bar={prop2}
/>
```
