Fix dll linkage issues on MSVC
Disable C4251 warning on windows

    class 'std::basic_ofstream<_Elem,_Traits>' needs to have dll-interface

According to http://msdn.microsoft.com/en-us/library/esew7y1w.aspx

  C4251 can be ignored if you are deriving from a type in the Standard
  C++ Library, compiling a debug release (/MTd) and where the compiler
  error message refers to _Container_base.
debugStream: Build and install in the right directory

Since iCub is compiled static on MSVC and debugStream library is static
or more likely dynamic depending on YARP, this will avoid to extend
the PATH variable, since bin should be already there.
Cleanup and small visibility fixes

* Log::LogType is now private (no reason to have it public, since it is
  accessed only by LogStream that is a friend class.
* Deprecated ConstString methods are back to public.
* Deprecated ConstString methods no longer have a function body. They
  were pure virtual before, so no reason to add a body).