Allow custom avatar sizes

This pull request allows users to create custom avatar sizes, or to override the default sizes, as follows:

```javascript
Avatar.setImageSizes({
  'large': 80,
  'mySize': 100
});
```

The default sizes are `large`, `small` and `extra-small`, and if no size option is passed then 50px is used.

When over-ridding the default sizes, or when creating new ones, the new CSS definitions are injected into the DOM dynamically to create new style rules. They are *not* added to each individual avatar element.

Note that this modification is 100% backwards compatible with existing code based on the latest version.

Ugh, that's embarrassing. :/