[WIP] improve Gridsearch

[dswah]

• [x] gridsearch should allow randomsearch-like behavior: ie pre-specify a grid for all lams, without relying on combine() do find the cartesian product
• [x] gridsearch should raise ValueError if grid dimensions cannot be inferred correctly
• [ ] gridsearch allows parallelism
• [ ] n_jobs is argument
• [x] gam.sample() uses multiple bootstraps with random-search
• [ ] add randomsearch() method?
• [x] intercept has lam=None, prints blank in summary
• [x] tests