Added status check functions

[stevepatter]

I've added HTTP status check functions that I've found useful, and may make it simpler to script around the curl library.

Now, a user can run

$curl = new Curl();
$curl->get('http://www.github.com/');
if ($curl->isSuccess()) {
    // handle curl response
} else {
    throw new Exception('failed to fetch content');
}

rather than having to check for specific http status codes. Of course, they still have access to the full status code should that be required.

[stevepatter]

I've adjusted the code for PSR-2 standards & it now passes PHP5 tests on Travis. Looks like the PHP7 & HHVM setup might be misconfigured.

[amouhzi]

Thank you