[ELY-4] Implement Unix MD5 Crypt

Implemented Unix MD5 Crypt and added some tests. Note that this implementation currently only handles an ID value of "1". Should "md5", the ID value representing the Sun MD5 Crypt algorithm, also be supported? (Just noticed that "md5" was referenced in the [identifyAlgorithm](https://github.com/wildfly-security/wildfly-elytron/pull/8/files#diff-89ce0408061333fe4d760f7738f33ebeR65) method in #8.) If so, I'll work on adding that but wanted to get some feedback in the meantime.

https://issues.jboss.org/browse/ELY-4