Implement apply command for applying diff patches

[ramsey]

Implement the apply command to provide the ability to apply diff patches to the working tree.

This PR also includes the commit from #28. I wanted to submit them separately for separate review, but if you look at the total changes in this PR, you'll see the changes for both commits, since #28 is required to make this work.

Example

In this example, you should be able to get an idea of the direction I'm heading in order to implement the CaptainHook feature I proposed here: captainhookphp/captainhook#122

use SebastianFeldmann\Git;

$repo = new Git\Repository(__DIR__);
$diff = $repo->getDiffOperator();

// Create patch from working tree.
$patchFile = tempnam(sys_get_temp_dir(), 'CaptainHook');
file_put_contents($patchFile, $diff->getUnstagedPatch());

// Reset working tree.
exec('git checkout -- .', $output, $exitCode);

$applied = true;

try {
    // Apply patch back to working tree.
    if (!$diff->applyPatches([$patchFile])) {
        if (!$diff->applyPatches([$patchFile], true)) {
            $applied = false;
        }
    }
} finally {
    if ($applied) {
        fwrite(STDOUT, 'Cleanly applied patch file to working tree: ' . $patchFile);
    } else {
        fwrite(STDERR, 'Could not cleanly apply patch file to working tree: ' . $patchFile);
    }
}

[sebastianfeldmann]

Looks great!