Release tar

Use the assembly plugin to create the tachyon release tarball.

I don't see anything in the repo that creates the tar, so assuming `tar zcvf ...` is used to create it.  Using the assembly plugin, we have a bit more control over what the tar looks like.

This is what the current tar looks like:

```bash
$ ls
bin     client  conf    core    libexec src
$ ls client/target/*
client/target/tachyon-client-0.6.0-SNAPSHOT-jar-with-dependencies.jar
$ ls core/target/*
core/target/tachyon-0.6.0-SNAPSHOT-jar-with-dependencies.jar
$ ls -1 src/
LICENSE
README.md
bin
client
conf
core
docs
libexec
pom.xml
tachyon-assembly
```