issue #55: add get rrset by name and type command to cli

for issue #55

This allows the cli to get a record set by name and type.  no output is returned if that set doesn't exist.

Example:
```
$ denominator -p mock record -z denominator.io. get -n www1.denominator.io. -t A 
www1.denominator.io.                              A      3600  1.1.1.1
www1.denominator.io.                              A      3600  1.1.1.2
```