Add support for multiple recipients encryptation

[rmdir]

This pull request add the encrypt_multi function, that allow encrypting for multiples recipents

[blaix]

Instead of a new function, how about letting encrypt accept an array by using the Array function?

irb(main):002:0> Array("hi")
=> ["hi"]
irb(main):003:0> Array(["hi"])
=> ["hi"]

[blaix]

Also, can you add a test for encrypt accepting multiple recipients?