Remove unity header app.h:

app.h is removed and new includes are inserted as needed to compile.
[EDIT] Add app modules to classic build
[WIP] Fix classic build
Include BeastConfig.h in every .cpp
Fix static initialization bug with backends
Tune SConstruct
Add core/ to nounity targets
Add crypto/ nounity target
Add json/ nounity target
[FOLD] Fix compile
Add net/ nounity target
Add overlay/ nounity target
Add peerfinder/ nounity target