Allow marking of single edits as bot edits

This lets you do things like:

```ruby
    require 'media_wiki'
    api = MediaWiki::Gateway.new 'http://mywiki.com/api.php'
    api.login 'Bot account', 'password'
    api.edit('Sandbox', 'This is a bot edit', {:bot => true, :summary => 'bot edit'})
    api.edit('Sandbox', 'This is not a bot edit', {:summary => 'not bot edit'})
```

To do this otherwise, you'd need to keep two separate instances of `MediaWiki::Gateway` around for edits and non-bot edits respectively. This is impractical and annoying, especially when you are writing a bot which needs to leave notifications on user's talk pages (and want these edits to appear in RecentChanges).