API changes

- User now takes an additional initializer argument, server, which is the relevant client instance.
- User now inherits Target.
- New methods for Target: msg, notice
- New methods for User: ping
- Deprecated the different "unescape" functions. This can be done by BeautifulSoup instead, as it should be.
- General improvements to the Twitter fetcher.
- General improvements to the YouTube fetcher.
- Ping response is now sent by notice.