Enforce apropariate amount of warnings and treat them as errors about radiostream

[khrynczenko]

Because dependencies are added in cmake with add_subdirectory, added compiler option propagate to them as well. This is unfortunate because /WX or -Werror will error not only on problems in RadioStream but also in POCO, nana etc.
Maybe we could change CMakeLists.txt so that dependencies are added with ExternalProject_Add command instead. This would allow us to set seperate options for us and them.

[khrynczenko]

So the flags were added in #74. Unfortunately the header files of the external project still gave errors because MSVC doesn't provide a way to specify inlude directories that are external (like gcc isystem <path>). Only way to do that for now is to surround external includes with

#pragma warning (push, 0)
// external includes
#pragma warning (pop)

Anyway it works and is resolved for now.