Added Nato Alphabet

Adds support for the nato alphabet and callsigns.

http://en.wikipedia.org/wiki/NATO_phonetic_alphabet#Letters

For example:

```
Faker::NatoAlphabet.code # => "OSCAR"
Faker::NatoAlphabet.callsign # => "ALPHA-CHARLIE-TANGO"
Faker::NatoAlphabet.callsignify("?-?-#-?") # => "XRAY-FOXTROT-SIX-ROMEO"
```