Parse urls using POSIX regular expressions

Thanks for fixing #4 (closed)! I had some free time over the weekend so I decided to see how hard it would be to write a replacement url parser using POSIX regular expressions. This is the result. I don't know if it's something you would be interested in or not, so I figured I would just submit it.

I've replaced the strtok_r based code in url_parser.c with regcomp()/regexec(). The match expression is URL_REGEXP, defined in url_parser.h. I've included a simple test suite in test_url_parser.sh.

I've marked up url-parser-c/url_parser.h so that you can use Doxygen (http://www.doxygen.nl/) on it.