Use the branch mvn-repo as a remote maven repository

The purpose of this PR is to allow people to use this github repo as a Maven repo too, so that instead of having to download it and run `mvn install` by hand, they can just clone any repo with the following code and the usual slick-2d dependencies in its `pom.xml` and it would download all the dependencies from it without any problem:

```XML
<repositories>
    <repository>
        <id>slick2d-mvn-repo</id>
        <url>https://raw.github.com/nguillaumin/slick2d-maven/mvn-repo/</url>
        <snapshots>
            <enabled>true</enabled>
            <updatePolicy>always</updatePolicy>
        </snapshots>
    </repository>
</repositories>
```
    
In order for people to be able to do that, you will personally have to take a couple steps, since you are the owner of the repo:

First, you will need to add your authentication for github in your maven's `settings.xml`, as the following:
```XML
<settings>
    <servers>
        <server>
            <id>github</id>
            <username>Your-Github-Username</username>
            <password>Your-Github-Password</password>
        </server>
    </servers>
</settings>
```
or, alternatively, (although I have not tried this one)
```XML
<settings>
    <servers>
        <server>
            <id>github</id>
            <password>OAuth-2-Token</password>
        </server>
    </servers>
</settings>
```
Next, you only have to run `mvn clean deploy`, and a branch called `mvn-repo` will be created (only in the Github remote), holding all the artifacts from all the modules.

As for the mvn-repo folder that gets created in the root folder, it gets deleted by just calling `mvn clean`, and in case you forget to delete it, it is ignored too.

Hope you take into account how much this would ease the use of this repository, and thanks for making the effort of putting this repo together in the first place.