In order to get rid of the namespace pollution, all "using namespace"
statements have been removed from header files. Where needed, they
were moved into the implementation files. Further work is need to get
rid of the std namespace pollution coming from headers in the
src/syncevo directory.
Removed the temporary common.h header and distributed the includes
across the individual source files. Attempted to have as few as
possible includes in the .h files, preferring to forward reference
where possible.
