add mbed_lpc1768 hwtimer stub

closes #586
refactor and clean up native Makefile
valgrind target
added license headers and doxygen file information

plus replaced tabs with spaces
added @author fields

authors found in:
https://github.com/RIOT-OS/RIOT/commits/master/core/cib.c
http://ukleos.org/projects/ukleos/repository/revisions/master/changes/core/cib.c
Merge pull request #593 from mehlis/cib-license

cib.c: added @author fields
addressed comments from @mehlis
Merge pull request #596 from LudwigOrtmann/mbed_hwtimer_stub

add mbed_lpc1768 hwtimer stub
added README that explains how to use the rpl_udp example application
Merge pull request #605 from Lotterleben/documentation

added README that explains how to use the rpl_udp example application
added LGPL header and doxygen headers
Merge pull request #619 from mehlis/sys-license

added LGPL header and doxygen headers
Merge pull request #606 from OlegHahm/license_issues

added license headers and doxygen file information
added LGPL header and doxygen headers
added LGPL header and doxygen headers
Merge pull request #617 from mehlis/examples-license

added LGPL header and doxygen headers
Merge pull request #601 from LudwigOrtmann/valgrind_target

valgrind target for native
Merge pull request #618 from mehlis/test-license

added LGPL header and doxygen headers
Add USEPKG variable for Makefiles

Currently pkg/USING says one should use
`EXTERNAL+=$(RIOTBASE)/pkg/<pkg_name>` to enable PKG modules.
Using this line the PKG will be compiled but not linked.

This change adds a USEPKG variable to be used like
`USEPKG += <pkg_name>`, which looks less clumsy and gets the PKG linked
in the binary.
Use include path convention for PKGs