handle truncating to zero words

## Issue Summary
The code incorrectly handles numeric zero when truncating.  String quoted zero ("0") works.

## Steps to Reproduce
```
$ node
> downsize = require('./index.js')
[Function]
> downsize("<p>this is a <strong>test of word downsizing</strong></p>", {words: 1})
'<p>this</p>'
> downsize("<p>this is a <strong>test of word downsizing</strong></p>", {words: 0})
'<p>this is a <strong>test of word downsizing</strong></p>'
> downsize("<p>this is a <strong>test of word downsizing</strong></p>", {words: "0"})
'<p></p>'
```

## Technical Details
Downsize Version: master (latest commit: ed2d74dcb7031c3a57b43b73197e40809fa3912e)
Client OS: Mac OS X
Node Version: 0.10.12