Properly calculate divisors in rule-based number formatters

Currently in master, this code produces the wrong result:

```js
var cldr = require("./lib/cldr");
var renderer = cldr.extractRbnfFunctionByType("en");
renderer.renderDigitsOrdinal(132);  // > "132th"
```

Clearly this should be "132nd" instead :)  This pull request fixes the problem by correctly calculating the divisor in the `if (specialChar)` code path.  From the [docs](http://grepcode.com/file/repo1.maven.org/maven2/com.ibm.icu/icu4j/3.4.4/com/ibm/icu/text/RuleBasedNumberFormat.java):

> ... To calculate the divisor, let the radix be 10, and the exponent be the highest exponent of the radix that yields a result less than or equal to the base value. ... If the exponent is positive or 0, the divisor is the radix raised to the power of the exponent; otherwise, the divisor is 1.

I've been working on a Ruby port of your excellent rule-based number formatter implementation - time to give back!