DATAFU-36 Add doap file

https://issues.apache.org/jira/browse/DATAFU-36
DATAFU-57 Source tarball generation and gradle bootstrapping

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

Note: This includes a lot of Gradle build code taken from Apache Samza.

Added the Gradle bootstrap task to be used when building from the source release.  This is from SAMZA-283.

Created a sourceRelease task to build the source tarball.  There is also a signSourceRelease task to build a signed version.
The source release does not include the gradle jar.  This is why the bootstrap task is necessary.

Created RELEASE.md for release instructions on how to create the source tarball.

Did some preparation for a binary release.  The artifacts datafu-pig and datafu-hourglass can be installed to the local maven repo using the 'install' task.

I also fixed all the javadoc issues, which caused build errors when I added the nexus plugin.  These were numerous.

Add org.gradle.jvmargs to gradle.properties