Added lib and obj directories.

This was giving errors when compiling after a fresh clone of the repository.
Fewer compiler warnings.

Otherwise, there were a couple of warnings like this: `./include/expression_sim.h:423:7: warning: control may reach end of non-void function [-Wreturn-type]` because the function `bool increase_iteration(ExprScopeT* scope)` might have ended without a return value.
Added basic install target to Makefile.

Right now this only works for the non-mpic++ variety: In that case the installation procedure is 'make lib' and then 'sudo make install'. Later a flag in the Makefile, or better yet a configure script, should make sure the installation works even if mpic++ is not installed.
Merge pull request #8 from olavolav/minor_fixes

Minor fixes