add network::Message

as base class for response messages other than data payload
add Network::Error for managing API errors
separating public headers in own dir

moved public header files into own directoy tree separating them from
private header files and implementations
adjust Doxyfile to new directory layout
namespace cleanup

- renamed namespace 'oQtocat' -> 'oqtocat'
- renamed namespace 'githubapi' -> 'ghapi'
- renamed namespace 'network' -> 'netio'

Shorter names seem better
tweaking cmake config

- add install directives
- executables (such as tests) will be build in )/bin(/tests)
further tweaks for CMake