Create cipher.rb

Adding a new filter. Cipher goal is to provide on the fly event content encryption and decryption. Beware of CPU cost.
Update comments on cipher.rb input
Reinit cipher in case of error.

cipher object reinitialization prevent incorrect state of the object
Fix key size option

oops key size was fixed...
Merge pull request #493 from saez0pub/LOGSTASH-Filter-Cypher

Logstash filter cypher