modified Serializer.hh to make it more type-safe and to fix warnings/potential problems with types of width <= 8 bit

- avoids undefined shift of 8 bit in read_int and write_int if width of type is equal to 8 bit
- used fixed width variables where possible
- addresses issues eudaq/eudaq#54 and eudaq/eudaq#12

(cherry picked from commit 2485a28b70524bfd51667f47a7de7286a3d405cf)
workaround for compilation issue with Clang version 3.5 (Apple Clang version 4.2) in Serializer.hh

- adds explicit conversion of eudaq::Time to timeval via GetTimeval() method in Time.hh, and used in Serializer.hh
- fixes issue eudaq/eudaq#53

(cherry picked from commit d310746349f5a6dd8cbe8be47a691ffbfcc747df)
Merge pull request #56 from hperrey/serializer_fixes

Serializer.hh fixes