Added hunter_cmake_args function.

This allows a project's hunter.cmake file to reuse one of the standard CMake schemes, but still customise the CMAKE_ARGS used without polluting the config.cmake file.
Update copyright.
Give priority to CMAKE_ARGS set via config.cmake.

We were overriding all CMAKE_ARGS set for a package when calling `hunter_cmake_args`.  That meant that CMAKE_ARGS specified in config.cmake weren't being honoured.

This fixes that by changing `hunter_cmake_args` so that it prepends its arguments.  Because config.cmake (containing calls to `hunter_config`) is processed before the packages' hunter.cmake files (containing calls to `hunter_cmake_args`), config.cmake overrides hunter.cmake.
Unit test to ensure `hunter_cmake_args` prepends its arguments.

Conflicts:
	.travis.yml
Removed unused include.
Use common test include file to locate Hunter code.

Conflicts:
	tests/issue/24/integration/CMakeLists.txt
Merge pull request #19 from alamaison/hunter_cmake_args

Added hunter_cmake_args function.