Mip

small fix: objective_value and objective_bound now return None if no optimization was performed

- return None for dual information if not available
- updated cbc binaries with fixed to mipstart and incumbent callback

- improved performance for lazy constraints and cuts: now cbc assumes that cuts and lazy constraints are globally valid, just like in gurobi
- fix max_gap model property
- small doc improvements

- fix limit of max_solutions in cbc when specified as parameter in optimize

fix for lp_method in CBC

- selection of which linear programming method (Primal Simplex, Dual Simplex or
Barrier) will be used to solve the root node relaxation (lp_method attribute
of Model)

- read/write property RHS in Constraint

- do not throw exception when querying an unavailable solution: just return
None instead, to fix problem in some debuggers

- improved documentation - specially for the examples

- pytest for tests - thanks pmla

- continuous integration with travis