Fixing issue #1

Hi,

I have made some changes that enable members to choose whether to use Gravatar or not. I also added a checkbox on the plugin's settings page which, when checked, forces every member to use Gravatar.

If a member hasn't uploaded an image he/she will get the Gravatar regardless of his/her settings.

In addition, I have also added a tiny bit of JavaScript to members' Settings page that will hide their original (uploaded) avatar and/or the link to gravatar.com when appropriate. It also replaces the Gravatar image with the uploaded image when a member unchecks the "Use Gravatar instead of your own image" checkbox, because I felt it was confusing when it showed the Gravatar.

Three new translatable strings have been added:

```php
$definitions["Force all members to use Gravatar"] = "Force all members to use Gravatar";
$definitions["Use Gravatar instead of your own image"] = "Use Gravatar instead of your own image";
$definitions["Note: This setting has no effect if you haven't uploaded your own image."] = "Note: This setting has no effect if you haven't uploaded your own image.";
```

I'm sure you want to suggest some improvements to them, as my use of the English language has become somewhat sloppy in recent years. Not enough practice, I'm afraid.

A question regarding line 139 in `plugin.php`: Should `gravatar.useGravatar` be `Gravatar.useGravatar` instead?