Add Travis building support

For this to work, you have to complete theses steps :

1. Sign in on travis-ci.org with your Gtihub credentials
2. Enable Travis-CI support for your repository
3. Optionally check "Build only if .travis.yml is present"
4. Send a mail to support@travis-ci.com asking to enable the OS feature on this repository
5. Install travis cli (sudo gem install travis)
6. Update the OAUTH token (api_key section) by issuing "travis setup releases".

Once this setup is done, the build should be triggered each time a commit is pushed. Creating a new release using the Github webpage will trigger artefacts building and uploading.

Note : Win32 cross-compilation left as an exercise to the reader.