Support github release

[luckyrandom]

This pull request add support for github release, which was requested in #124. Please feel free to move the following doc to any place such as wiki. You may check https://travis-ci.org/luckyrandom/r-travis-test/builds for a working example.

Release package on github release

Set up

• Create a new token in github application settings
• Encrypt the token following
  the document on travis-ci,
  by running the following command in the project directory

travis encrypt GH_TOKEN=your_token_created_on_github

• Add the following sections to .travis.yml

-env:
  global:
    # GITHUB_TOKEN
    - secure: "your_encrypted_token"
after_success:
  - ./travis-tool.sh deploy ./packagename*.tar.gz

Recommend workflow

When a package is ready, bump the version, and put [try deploy github] in a single line of commit comment. Then the deploy script will create release and upload pkg automatically. In detail, the deploy script will create a release on github if the build pass, which will trigger another build on Travis-CI, in which the deploy script will upload the package.

Alternatively, you can also trigger the deploy script to upload package by,

• Create release on github manually
• Push a tag to github, with the tag name similar to v2.3.2

[krlmlr]

Probably luckyrandom should be changed to craigcitro here, at least after merging.