Merge pull request #7 from bchretien/master

Factorize code for tests.
Add 7th problem
schittkowski/problem_6.cc: fix test case name
Add 8th problem
Add 9th problem
Add 10th problem
Add 11th problem (draft, compilation disabled for now)
Add 12th problem
Add 13th problem
Add 14th problem (draft)
Add 15th problem (draft)
Use user-supplied tolerance to BOOST_CHECK_SMALL.