Adding support for rule-based number formatters

The CLDR data set contains definitions for spelling out numbers in text.  Collectively they're referred to as "rule-based" number formatters.  The definition files contain groupings, groupings contain rule sets, and rule sets contain rules.  Here's an example of what they're capable of:

**English**
Ordinal of 123 => "123rd"
Ordinal Spellout of 123 => "one hundred twenty-third"

**French**
Spellout of 199 => "cent-quatre-vingt-dix-neuf"

**German**
Spellout of 2323 => "zwei­tausend­drei­und­zwanzig"

etc etc

Check out the `rbnf` branch of [twitter-cldr-rb](https://github.com/twitter/twitter-cldr-rb) for an implementation :)