enable CDash

the most fun part of this was to find out that `include(CTest)` was
missing in CMakeLists.txt. The dropsite is

http://www.opm-project.org/CDash/index.php?project=opm-parser
Merge pull request #219 from andlaus/cdash_enablement

enable CDash
Added Box() class to book keep the indices in a box.
Added constructor for sub box.
Added Box::equal() comparison function.
Added BoxManager() to keep track of the currently active input box.