gnuplot-commands: add some code coverage.
selection-by-id: add throw check in test.
problem.cc: check exceptions in test.
polynomial.hh: fix typo in doc.
travis.yml: add missing GH_REPO.
util.hh: add a copySparseBlock function.
Remove exception specifiers.

Fix #59
Merge pull request #60 from bchretien/exception_specifications_removal

Remove exception specifiers.
Remove another exception specification.
include/roboptim/core/problem.hxx: fix mistake in error message
gnuplot.hh: add clear command.
gnuplot: handle multiline comments.