Travis CI: build/update Doxygen from separate script

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.github.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.