Python 3 compatibility update

Hello there folks, this pull request relates to issue https://github.com/cobrateam/splinter/issues/165

**Update**: This pull request is now ready to be merged when you're ready :smile: 

## Python 2.7 ##

- **Django**: OK
- **Zope**: OK
- **Chrome**: OK
- **PhantomJS**: OK
- **Firefox**: 1 error, 1 failure (which were present before my work)
- **Remote**: OK

## Python 3.3 ##

- **Django**: OK
- **Zope**: N/A dependent library not currently Python 3 compatible
- **Chrome**: OK
- **PhantomJS**: OK
- **Firefox**: 1 error, 1 failure (which were present before my work)
- **Remote**: OK

All questions are welcome :smile: 
Fotis