Support github release

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](https://github.com/settings/applications)
- Encrypt the token following
  [the document on travis-ci](http://docs.travis-ci.com/user/encryption-keys/),
  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

The package was built before downloading github-release.rb, so we don't need to add it to `.Rbuildignore`.