Implements #12
* Implement Stable Models based on lazy evaluated iterators
* Adjustment of the internal computation of the grounded interpretation
* Update build.rs to replace "@" in test-instance names with "at"
* Implement de-/serialization of the adf (in OBDD representation) in library
* Adjust tests
* Add more style-restrictions to compiler
Utilised quickcheck, assert_cmd, predicates, and assert_fs for better
tests of low-level functionality as well as cli-testing.
Signed-off-by: Stefan Ellmauthaler <stefan.ellmauthaler@tu-dresden.de>
