Merge branch 'master' of https://github.com/vgvassilev/clad

Conflicts:
	lib/Differentiator/DerivativeBuilder.cpp
Fix cloning of statements.
The references of IfStmts are now updated and Sema is used for visiting return
stmts.
Test results are evaluated and out files are generated.
No need for updateReferencesOf to return Expr*.
Making it return void.
Merge branch 'master' of https://github.com/vgvassilev/clad
Remove useless code.
Merge pull request #5 from mvassilev/master

Fix cloning of statements. No need for updateReferencesOf to return Expr*. Improve tests.