feat: use selective logging

The following will show all logs from all modues:

    ./dReal --verbose <xxx.smt2>

It's possible to see the logs from a selected modules. For example,

    ./dReal --vmodule=icp_solver=3,nra_solver=2 <xxx.smt2>

will show all logs (FATAL, ERROR, WARNING, INFO) from icp_solver module,
and FATAL, ERROR, WARNING logs from nra_solver module.

Please see util/logging.h and glog documentation[1] for more details.

[1]: http://google-glog.googlecode.com/svn/trunk/doc/glog.html