Add override to force rendering all labels on the X Axis.

Useful when individual X values are important, and plentiful.

calling:
```javascript
char = new xChart('bar', data, '#selector', {'showAllXTicks': true});
```

example output:
![graph](http://f.cl.ly/items/163w2K2b0V3S3N0c2t2E/Screen%20Shot%202013-11-25%20at%208.27.40%20PM.png)