C++11 compatibility

These changes enable to compile RIOT using GCC 4.7.3 using g++ with gnu++11 features.
It is tested to compile to a working native binary with the default-native project.
The changes are mostly not invasive to be fully compatible with the recommended gcc with gnu99 features.

Compiling for the MSB-A2 is also working with the gnu++11 changes. 
-There is an open issue with optimization level and the sht11 sensor using GCC 4.7.3. 
  compiling with Level -O2 prevents RIOT to boot on the MSB-A2, Level -O0 can be used as workaround by now.