ported all Checks from LiipMonitor (fixes #3)
Cleanup

- Refactor class names (remove "Check" suffix).
- Remove Check\WritableDirectory as it overlaps Check\DirWritable.
- Replace occurrences of "Disc" with "Disk".
- Remove getName() methods, as they do not add any value to default implementation of AbstractCheck::getName() and ::getLabel().
Add better error handling for Check\Memcache.
added suggest for sensiolabs/security-checker