Instead of fiddling with the non-working GIT_ASKPASS/core.askpass
setup, simply store the required Git credentials directly in
~/.git-credentials, which is used by Git's "store" credential helper.

Works on https://travis-ci.org/pah/rapidjson.

Avoids messing with global config when running the script
manually on a developer machine.

This reverts commit 62250923.

This reverts commit ded7dc72.

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.