Manage options in the facade. Avoid dynamical instantiation of terms.
Add "onHead" and "onBody" methods. They are called by the parser at the end of heads and bodies, respectively.
Merge branch 'master' of https://github.com/veltri/DLV2

Conflicts:
	src/DLV2Facade.cpp
	src/aspcore2.l
	src/main.cpp
	src/util/Options.h