Refactored the plugin code and simplified installation

Refactored the plugin code and XMPP pre-binding:
- Simplified XML request sending and removed duplicated code
- Added debug output and error reporting
- Send separate restart command before bind
- Encrypt password when stored in session 
- Use more Roundcube plugin API features
- Moved config template to config.inc.php.dist
- Made configuration options more flexible (no functions required)
- New option to use chat without prebinding
- Maintain a minified version of converse.js for easy installation
- Describe how to build that vendorized version of convese.js
- Updated to latest converse.js version from the official repository
- Support skin-specific styling of the chat window
- Support localized texts

I plan to add more improvements and a better UI integration. So stay tuned...