move looksLikeLatLonPair into lib.php, basic PHP test suite using phpunit

We were looking into supporting more coordinate formats, similar to https://github.com/twain47/Nominatim/issues/147 and added that to Nominatim.

The more significant change is that I added a basic phpunit setup which helped me a lot with debugging. I don't expect it to be part of the installation instructions (similar to how the python/lettuce test suite isn't) but more tests are always good. PHP is not my favorite language and I've used phpunit only twice in the past. I hope the code style and setup matches the rest of Nominatim.