Add support for Github based maven repo

Hi, I added support for using Github as a Maven repo as suggested [here (please read me)](http://stackoverflow.com/questions/14013644/hosting-a-maven-repository-on-github).

After you merge the request, you need to do the followings.

#### Adding Github Authentication 
Add the following code to your `~/.m2/settings.xml`. The id `github` is used by `<github.global.server>github</github.global.server>` in `pom.xml`. The password is used to commit artifacts to github repository.

```
<settings>
  <servers>
    <server>
      <id>github</id>
      <username>YOUR-USERNAME</username>
      <password>YOUR-PASSWORD</password>
    </server>
  </servers>
</settings>
```

#### Deploy
Run `mvn clean deploy` to deploy artifacts into Github `mvn-repo` branch.

#### Reference the Repo
Users need to add the following code to reference this repo

In Maven
```
<repositories>
    <repository>
        <id>antlr4-mvn-repo</id>
        <url>https://raw.github.com/antlr/antlr4/mvn-repo/</url>
        <snapshots>
            <enabled>true</enabled>
            <updatePolicy>always</updatePolicy>
        </snapshots>
    </repository>
</repositories>
```

In Gradle
```
maven {
    url "https://raw.github.com/antlr/antlr4/mvn-repo/"
}
```