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
* Implemented a nom-based parser to read the adf
* Grounded semantics (naive) re-implemented
* Docs added
* Updated Cargo.toml with more Manifest information
* Version update
* Added sort-methods to the parser, so the var-order can be adjusted
* Added sort functionality to the main function
* Added adf-instances as a submodule to the res-folder
* Added README information for the extended integration tests
* Rewritten main-function
Closes#3
