Improve support for out of source builds

It is good practice to support out of source builds (`mkdir build; cd build; cmake ..`) as opposed to in-source builds (`cmake .`). This way you can easily delete the temporary build files by deleting the build folder. Also, the `.gitignore` file can be simplified since you only need the `.gitignore`.

In this pull request, I change many references to `${CMAKE_SOURCE_DIR}` to `${CMAKE_BINARY_DIR}`, so that temporary files are created in the build folder.

Also, I was having trouble finding the urdf libraries, so I fixed that in FindDARTExt.cmake.in

I also removed a CMakeCache.txt file (#77), since that should not be in the source repository.