Setting up Travis CI
Commit before renaming Solver to Model.
Big messy commit …

… including documentation skeletons, tests and coverage, pretty advanced glpk_interface, rudimentary gurobi and cplex alternatives and a lot more
This commit coincides with the first successful cobrapy FVA (flux_variability_analysis) running completely on top of optlang.
Snapshot for Ali (coincides with phantomas1234/cobrapy@770724e5a5c1995853be7ab5d59af6edcff78319).
Before merge with master.