Source tarball creation and gradle bootstrapping

[Matthew Hayes <ma...@gmail.com>]

Hi all,

I have created a patch that updates the Gradle build scripts to create a
source tarball that we can hopefully use for release.  This is based on
Jakob's work in SAMZA-283 and uses a lot of that code.  It also includes
some prep work for doing a binary release.  There are now tasks that can
install to a local maven repo.  I've done some instruction cleanup in the
README.md and also added a RELEASE.md, mostly derived from the one in Samza.

Please review:

https://issues.apache.org/jira/browse/DATAFU-57

https://reviews.apache.org/r/24215/

Note that there are many files touched in this change.  The reason is that
I cleaned up all the javadocs, which were resulting in build errors and
warnings.  Changes to these files are individually pretty minor and should
be quick to review - there's just a lot of them.

Thanks!
Matt