Implement support to verifySSN operator into libmodsecurity

[zimmerle]

Operator verifySSN needs to be implemented inside libmodsecurity.

The documentation of the operator can be found here:
https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#verifySSN

The file already exist in the repository:
https://github.com/SpiderLabs/ModSecurity/blob/libmodsecurity/src/operators/verify_ssn.cc

[zimmerle]

Done. a7f7532.