Add Check\DiskFree with tests and README.

### Check\DiskFree

Check if there is enough remaining free disk space.

The first parameter is the minimum disk space, which can be supplied as integer (in bytes, i.e. `1024`)
or as a string with a multiplier (IEC, SI or Jedec, i.e. `"150MB"`). The second parameter is the path to check -
on *NIX systems it is an ordinary path (i.e. `/home`), on Windows systems it is a drive letter (i.e. `"C:"`).

````php
<?php
use ZendDiagnostics\Check\DiskFree;

$tempHasAtLeast100Megs  = new DiskFree('100MB', '/tmp');
$homeHasAtLeast1TB      = new DiskFree('1TiB',  '/home');
$dataHasAtLeast900Bytes = new DiskFree(900, __DIR__ . '/data/');
````