Update feature list

The list of features on gajim.org is a bit outdated and needs an update.

This should also include improvements regarding which users are adressed. Right now it adresses advanced users. We should highlight the features wanted/used by the majority of users.

What I think should be highlighted:

• Message synchronization (between multiple clients)
• Image/Video/File sharing (even with offline users)
• Encryption (types)
• Emoticons

Please add the ones I forgot to mention.

Right now the list is a bit cluttered with more or less important features. Restructuring and updating the list should help already.