[MESOS-1261:MESOS-1288]: Use maven to compile and package Mesos' Java files.

Applied
=======

[MESOS-1212]: Use maven to compile and package Mesos' Java files.

The Mesos Jar files are now generated through Maven. This update also
adds a _shaded protobuf jar_ that contains the _protobuf_ version _shaded_
inside the `org.apache.mesos.protobuf` namespace.

List of Java Artifacts.

* mesos-<Mesos Version>-javadoc.jar
* mesos-<Mesos Version>-shaded-protobuf.jar
* mesos-<Mesos Version>-sources.jar
* mesos-<Mesos Version>.jar

Where `mesos-<Mesos Version>-shaded-protobuf.jar` will be the dependency
that a _framework_ should use to avoid any conflicts of _protobuf_
versions that it might have with other _dependencies_. Not that
`shaded-protobuf` is the _dependency qualifier_.

Applied patch from:
    https://reviews.apache.org/r/20402/

ticket : https://issues.apache.org/jira/browse/MESOS-1212

[MESOS-1252]: Support ENV MAVEN_HOME on the build.

The build now supports the `MAVEN_HOME` environment variable which is
used to resolve the `mvn` executable. If no `MAVEN_HOME` is specified we
will use the one available in the `PATH`.

ticket: https://issues.apache.org/jira/browse/MESOS-1252
[MESOS-1261:MESOS-1289]:Fix Javadocs -- Executor.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- ExecutorDriver.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- Log.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- MesosExecutorDriver.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- MesosSchedulerDriver.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- Scheduler.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- SchedulerDriver.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- State.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- Variable.java
[MESOS-1261:MESOS-1289]:Fix Javadocs -- ZooKeeperState.java