new method: marginOfError

Hi @tblobaum

Thanks for this module!

### New method:
```
marginOfError(confidence)
```
 - developed it according to this formula: http://www.wikihow.com/Calculate-Confidence-Interval
 - used this library for calculating Z value distribution: https://gist.github.com/benrasmusen/1261977 (node.js: https://gist.github.com/pedrombafonso/8541077)
- [x] Added tests
- [x] Updated the documentation