set_default_settings renamed into osqp_set_default_settings (oxfordcontrol/osqp@085f719c369aa248788bf9770640a95678def528)

- add clang compiler for linux environment in .travis.yml

General refactoring

- rename setDefaultSettings() into resetDefaultSettings();

- write 'OSQPFindPackage.cmake'
- add OSQP dependency in 'OSQPWrapperConfig.cmake.in'

- write a simple QP problem test

- general refactoring of the 'setUpperBound', 'setLowerBound' and 'setGradient' methods in OptimizatorData.tpp

- implement 'size()' method in SparseMatrix class
- implement 'getSparseMatrix()' method in SparseMatrix class