fixed non-integral-type global static const variables

from Qt's documentation:

    Anything that has a constructor or needs to run code to be
    initialized cannot be used as global object in library code, since
    it is undefined when that constructor/code will be run (on first
    usage, on library load, before main() or not at all). Even if the
    execution time of the initializer is defined for shared libraries,
    you’ll get into trouble when moving that code in a plugin or if the
    library is compiled statically.
    
(https://qt-project.org/wiki/Coding-Conventions)
made API endpoints a struct with static members
add license info to new public headers
forgot to export Config and ApiEndpoints
enabled build of static library
extend and update Travis config

+ add further build variations to build matrix:
  - BUILD_TYPE={Debug,Release}
  - BUILD_SHARED={ON,OFF}
+ print out installed directory structure (including file sizes)
+ print out summed size of installed files