problem.hh: add support for arguments names.

Close #46
optimization-logger.hh: use arguments names if available.
optimization-logger.hh: add test + fix for non-differentiable functions.
[travis] Synchronize.
Miscellaneous cppcheck fixes.
optimization-logger.hh: improve tests (dense/sparse, differentiable/non-differentiable).
travis: allow failures for osx until #67 is solved.
Merge pull request #66 from bchretien/feature/arguments-names

Feature: set arguments names