Add Git build info when we build a distribution

[kimchy]

Would be nice to have git data (like commit hash, UTC time, ...) of a build of ES.

Initial thoughts are we should have a Build class similar to Version class (we don't want to have this info as part of Version, since we serialize it on each request), that would read its info from a properties file that gets included in our jar file. We can use this plugin: http://mojo.codehaus.org/buildnumber-maven-plugin/ to do it potentially.

We should return the Build info on top of the Version info in the following cases:

• when doing GET /
• bin/elasticsearch -v
• when the server starts up