Symbols visibility

[EduPonz]

This discussion's intent is to define a better approach from symbols' visibility. It has sprouted from #88 and #89 by @roehling.
Important points to clarify would be:

1. Fix the problem of exporting the library exception classes, which right now are not exported per se, but in a member function by member function basis instead, since they inherit from std::exception which is not exported.
   For this case, Fix visibility of exception classes #88 proposed simply disabling the MSVC warnings, but does not seem like a solution to the problem either, so it was discarded.
2. Whether to set the -fvisibility=hidden and -fvisibility-inlines-hidden flags in GCC so that only the specified symbols are exported (similar to MSVC approach)