Serializer.hh fixes

Various fixes to the code in ```Serializer.hh```:
- make sure that variables of width <=8bit are handled correctly when shifting data in (fixes eudaq/eudaq#12)
- fixes compilation problems on Clang >3.2 (or Apple LLVM 4.2) (fixes eudaq/eudaq#53)
- addresses type-safety in the serializer code (addresses eudaq/eudaq#54)