Fixed doxygen comments, focused on file headers and group definitions

Hi @all,

as I am new to RIOT I will give my first shot at putting a pull request together: while looking through the code I tried to clean up some of the doxygen comments in RIOT, mainly focusing on bringing the doxygen module structure in order. I tried to unify the page header comments to the following style and order:
1. Copyright
2. Group assignment
3. File information
4. Authors

I tried to use the following blueprint for each (.h) file:
```c
/*
 * Copyright (C) 2013 Freie Universität Berlin
 *
 * This file subject to the terms and conditions of the GNU Lesser General
 * Public License. See the file LICENSE in the top level directory for more
 * details.
 */

/**
 * @addtogroup  core_util
 * @{
 */
 
/**
 * @file         clist.h
 * @brief       Circular linkes list
 * @author    Freie Universität Berlin, Computer Systems & Telematics
 * @author    Kaspar Schleiser <kaspar.schleiser@fu-berlin.de>
 */

// beautiful code

/** @} */
```

and for .c files:
```c
/*
 * Copyright (C) 2013 Freie Universität Berlin
 *
 * This file subject to the terms and conditions of the GNU Lesser General
 * Public License. See the file LICENSE in the top level directory for more
 * details.
 */

/**
 * @addtogroup  core_util
 * @{
 * @file         clist.h
 * @brief       Circular linkes list
 * @author    Freie Universität Berlin, Computer Systems & Telematics
 * @author     Kaspar Schleiser <kaspar.schleiser@fu-berlin.de>
 * @}
 */

// here comes the code
```

For group definitions that are not clearly traceable to a specific file I inserted doc.txt files into the module folders (e.g. RIOT/core/doc.txt, RIOT/sys/net/doc.txt, ...). The location and naming of these files is surely something we should talk about...