Adding multimodule parent pom with developers and contributors sections

With this pull request all the java modules build from the top level directory
```bash
$ pwd
genomics-tools

$ mvn clean install
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO] 
[INFO] Genomics API multimodule
[INFO] Genomics API benchmarker-java
[INFO] Genomics API client-java
[INFO] Genomics API mapreduce-java
[INFO] Genomics API readstore-local-java
...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] Genomics API multimodule .......................... SUCCESS [  0.324 s]
[INFO] Genomics API benchmarker-java ..................... SUCCESS [ 10.000 s]
[INFO] Genomics API client-java .......................... SUCCESS [  6.857 s]
[INFO] Genomics API mapreduce-java ....................... SUCCESS [ 38.177 s]
[INFO] Genomics API readstore-local-java ................. SUCCESS [  2.154 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
```
It does mean that you have to use JDK 1.8 to build all the modules since readstore-local-java requires it.

There is some inconsistency in the version numbers though:  multimodule, client-java, and readstore-local-java are version v1beta; mapreduce-java and benchmarker-java are version v1.

I have added only the bare minimum to the developers section, feel free to add more details if you wish.

See also issue #38.