Clingox

* fix printing of weight rules (39)
* fix printing of external directives (45)
* add function to remove function symbols from AST (41)

* make compatible with clingo 5.6
* add module to ease unit testing
* add pretty printing module
* extend AST module

* add a new module to reify programs (9)
* add a new function to reify atoms (10)

* fix dependency problem

* initial release