USER_AGENT_STRING -> CLIENT_STRING, and use ORG/APP_NAME to build.
Append platform infos to user-agent string.

Exemple of output:
MusicBrainz-Picard/1.2.0final0 (Linux-3.5.0-17-generic-x86_64-with-LinuxMint-14-nadia;CPython-2.7.3)
Update NEWS.txt
Merge pull request #190 from zas/useragent

Append system info to user-agent string, and build it from constants