[urban.py] Fixed and enhanced the module

I fixed and enhanced your urban.py and rather than putting the modified version into my own repo, I decided a pull request would be more appropriate. What I fixed? Well for example the original version would output a unicode error when you'd do `.urban ~ß€↑ħ→ß` so I fixed that among with some other things like when the definition had a unicode character in it, the module would output an encoding error.

Also here's a pic how it looks now:
![ss-2014-05-14-06 09 28](https://cloud.githubusercontent.com/assets/6899132/2966594/8662b5f4-db15-11e3-841c-27de1f3c21f7.png)