Cleanup the build system

This pull request changes the following:
* Removes all other build systems to create one build system that should work for everyone: CMake
* Moves all the windows stuff to the submodule `src/platform_windows`
* Adds the ENet library as a submodule to `src/libraries/ENet`
* Adds a travis file was also added.

This will need some testing by windows and mac users, as I was not able to test it on either platform.

Recommended usage on linux:
Only run this the first time: `(mkdir build && cd build && cmake ..)` after that: `(cd build && make install)`
You can replace `cmake ..` with `cmake -DCMAKE_TOOLCHAIN_FILE=../src/platform_windows/linux-toolchain-mingw.cmake ..` to cross compile for windows.