Wrap btsync executable to enable auto-generation of config if necessary

Changelog:

- Add hint about daemon-reload to post_install output
- Add command-line options support to config-script
  - The user can now modify the generated config file by specifying command
    line options such as --login, --device-name etc.
  - Use of the config script is now encouraged over copying
    /etc/btsync.conf. The config script is now also used to create
    /etc/btsync.conf.
- Improve config script help message
  - Add warning about password in cleartext
  - Split into a long and a short version
  - Clarify usage by removing ugly, unneeded parentheses
- Wrap btsync executable with btsync-wrapper.sh
  - Services now point to btsync-wrapper.sh instead of the plain btsync
    executable. The script creates the specified config file if it does
    not already exist.

Internal stuff:

- Add .gitignore