Add tests & basic functions for FizzBuzz

http://www.rustforrubyists.com/book/chapter-05.html
Drop some minor notes on output formatting strings

Cribbed from the *Rust for Rubyists* [FizzBuzz][1] chapter.

[1]: http://www.rustforrubyists.com/book/chapter-05.html
Migrate tests to native assert!()
Quick n' dirty FizzBuzz output from w/in main

It should be tested, but for now I'm just following along in *Rust for
Rubyists*
Use return value of if directly in println
Clean up main impl to be a little more compact

Noting that single-line if/else in Rust still requires the {} brackets.
Extract FizzBuzz logic, add tests
Implement pattern matching, removes need for is_fifteen

We can build up the is_fifteen/FizzBuzz case by pattern matching for
is_three and is_five both being true
Merge FizzBuzz tinkering, http://www.rustforrubyists.com/book/chapter-05.html