Add more test cases. Tests all numbers between 1-9999
Fix tests. Use mocha's assert instead of should.
Fix test case
Alternate fix for #1

This is a cleaner approach, imo, because the regexes are all in one
place.
Handle some words that start with a

ex. a half, a hundred, a thousand.
Merge pull request #4 from monicao/fix_a_hundred_a_thousand

Fix numbers that start with a