feat(*): add "--debug" option

add another option --debug which will activate DREAL_LOG_DEBUG. The idea
is to use DREAL_LOG_DEBUG most of places instead of DREAL_LOG_INFO.

We expect that users use "--verbose" option while dReal devs use
"--debug" option:

 - Using --verbose activates DREAL_LOG_INFO.
 - Using --debug activates both DREAL_LOG_INFO and DREAL_LOG_DEBUG.
test(nra/sk.smt2): add Stefan Kiefer's example (at Oxford)

Read: https://github.com/dreal/dreal/issues/15
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fixup debug/log message
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