Relaxations are no longer self existed techniques but part of the group of backend solvers. User may set relaxation solver as driven solver.
Make a list of relaxation solvers a part of list of backend solvers. The default backend solver that will be used to solve a single subproblem or entire problem now is called default solver.
Make a list of relaxation solvers a part of list of backend solvers. The default backend solver that will be used to solve a single subproblem or entire problem now is called default solver.
Add knapsack solver as relaxation solver.
Rename default_solver and relaxation_solver from OptimizationParameters to default_backend_solver and relaxation_backend_solver respectively.
Fix default logging format.
Merge pull request #4 from d2rk/fix_logging

Fix default logging format
Merge branch 'master' of https://github.com/d2rk/les into relaxations_support