add mbed_lpc1768 hwtimer stub

closes #586
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