Create cipher.rb

Adding a new filter. Cipher goal is to provide on the fly event content encryption and decryption. Beware of CPU cost.
support external MIB directory for snmptrap input
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...
dns filter: custom nameserver support
new: unix socket input
Merge pull request #493 from saez0pub/LOGSTASH-Filter-Cypher

Logstash filter cypher
Merge pull request #495 from nikicat/dns-custom-nameserver

dns filter: custom nameserver support
Merge pull request #496 from nikicat/unix-input

new: unix socket input
Merge pull request #477 from rasputnik/master

add "yamlmibdir" option to snmptrap input.
Merge git://github.com/logstash/logstash