Fixes #145. Feature/issue 145 print

Merge pull request #268 from stan-dev/feature/issue-150-raise-exception-for-tau-greater-than-or-equal-to-y

Fixes #150: raise exception for tau greater than or equal to y

Updating using statements for some tests. Fixes #385. 

include make/local in make/tests

Eigen 3.2.9 lib, finishes #362

Removing some unused comments.

range checks throw std::out_of_range

Fixes #269. symmetrize (inv_)wishart_rng

Removing some comments

grad_reg_inc_gamma: more accurate for large z

looks good to Sebastian, so I'm merging

Fixes #345. Feature/0345 gcc6 warns