deploy test-jar with maven

As discussed [here](https://groups.google.com/forum/#!topic/tachyon-users/Ky7BkdqOmgo), I would like to access some classes in the Tachyon tests for the tests of my own project.

Therefore, I suggest with this pull request to create a so-called [test-jar](http://maven.apache.org/plugins/maven-jar-plugin/test-jar-mojo.html).
After a `mvn package` you'll find the following jar files in `target/`:
```
-rw-rw-r-- 1 robert users  1847951 Mar  3 22:24 tachyon-0.5.0-SNAPSHOT.jar
-rw-rw-r-- 1 robert users 20842410 Mar  3 22:24 tachyon-0.5.0-SNAPSHOT-jar-with-dependencies.jar
-rw-rw-r-- 1 robert users    95690 Mar  3 22:24 tachyon-0.5.0-SNAPSHOT-tests.jar
```

If others want to use the test-jar in their project (for testing), they have to add the following dependency

```xml
 <dependency>
    <groupId>org.tachyonproject</groupId>
    <artifactId>org.tachyonproject</artifactId>
    <version>0.5-SNAPSHOT</version>
    <type>test-jar</type>
    <scope>test</scope>
 </dependency>
```