Add version info to Flume NG

A user should have a way to know which version of Flume is being used. This can be implemented by adding a 'version' command to the flume-ng shell script. For example:

flume-ng version

...would result in the git commit # that the Flume NG build is based on. This would help with tracking a user's problems with using Flume.

I imagine that the build process would need to create some kind of buildinfo file that can be referenced by the script... that's just one idea. I'm interested in what others think.

Hadoop has the same feature: "hadoop version".