Allow use of UK postcodes

This change allows the use of UK postcodes, and any other formats. It provides a simple fix for issue #69.

The only problem is that there's now no validation done at all, which means that someone could put in an incorrect address. However, you could technically do this before - you just had to provide a zip code, but not necessarily a correct one! Basically, there's some extra validation that could be done, e.g. checking the address with Google and handling the error case :hamster:

I also had to fix the generation of the `/coder_search` URL, as the parameters weren't being encoded correctly. This hasn't been a problem before, but UK postcodes contain spaces (because we love to be awkward), and it meant that rails kasploded. I've swapped out the string generation with Rails' URL helpers.

I'm open to suggestions! Thanks