Added create_issue event to the activity feed.

This adds the `CREATE_ISSUE` type. Any issue plugins that inherit from the `IssuePlugin` right now will automatically use this new feature.

The feature is implemented by collecting the required information after `IssuePlugin.create_issue` is called and then saving it in the `Activity.data` attribute as a `dict` with the keys:

- provider: The provider of the issue tracker.
- title: The title of the newly created issue.
- location: An URI to the created issue.

This `dict` gets passed to the format string in `ACTIVITY_ACTION_STRINGS` for formatting. This allows localization to take place without too much problems.

Comments and problems appreciated.