Add support for SRV records
Basic test for SRV
Remove zero-length checks from SRV Target value:

According to the RFC(https://www.ietf.org/rfc/rfc2782.txt), empty targets are legitimate and have meaning:

"A Target of "." means that the service is decidedly not
available at this domain."
Add test for a range of bad SRV values.
Merge pull request #12 from duedil-ltd/feature/srv-records

Support SRV records