In this patch, a script 'build/travis-doxygen.sh' is added to
build and push the Doxygen documentation to the GitHub pages at
https://miloyip.githib.io/rapidjson.

The script exits gracefully, if the build is requested for
 - a branch other than 'master'
 - a pull-request
 - a job (i.e. CI configuration) other than "1"

In case the "secure variables" are not available, only the final upload
is skipped, in order to allow testing of the script's basic functionality.

Update .travis.yml to call the script.