Publish Javadoc to GH Pages via Travis CI

[dexX7]

The Javadocs are build via Gradle and pushed to the gh-pages branch of this repository. The script is executed by Travis CI.

The variable $TRAVIS_BUILD_DIR is set by Travis CI, while $GIT_REPO_URL must be provided as secure environment variable (via Travis CI), for example in the following format:

GIT_REPO_URL=https://[personal_access_token]@github.com/user/repo.git

No documentation is generated for pull requests, and to avoid that the docs are generated four times, for each build target once, it is further restricted to the oraclejdk8 builds.

See also:

• https://help.github.com/articles/creating-an-access-token-for-command-line-use/
• http://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings
• http://docs.travis-ci.com/user/encryption-keys/ (alternative)

This resolves #14.