Fix strict PHP bug with getAnchoredUrl()

WebUrl::getAnchoredUrl() would return errors in strict mode because
strpos() might receive an offset that was too large for the given
string.
Switch to one writeable directory

pocket-knife will only modify the 'writeable' directory
Add logging system to handle uncaught exceptions and errors

All logged messages will be saved to 'writeable/logs/error.log'
Ignore contents of writeable directory
Add explanation on what files will be created
Fix bug with strict PHP

If a directory was already created, strict PHP would trigger an error