Create SelectFieldByName UDF

Given a field who's value contains a field name, and *, returns the value of the field referenced by the field name.

https://issues.apache.org/jira/browse/DATAFU-69

Signed-off-by: Matt Hayes <mhayes@linkedin.com>
DATAFU-68: SampleByKey can throw NullPointerException

https://issues.apache.org/jira/browse/DATAFU-68

Signed-off-by: Matthew Hayes <matthew.terence.hayes@gmail.com>
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/incubator-datafu