Refactored to take a single URL argument & provide more informative error messages.

nagios-check-http-proxy requires the user to break out the URL into protocol, host, domain and path. This seems unnecessary work and makes calling the script more cumbersome. Also, the error messages just let you know there's a problem. Not whether it's a proxy or target website issue.

This pull request:

* Parses URL for all the information it needs about the target website (i.e. just supply protocol://domain/path instead of separate arguments).
* Provides separate error messages for proxy unavailable and target website unavailable.