adding precious to improper.exe
fixing the make target for executables
removing improper model from list of .PRECIOUS
Merge pull request #153 from stan-dev/feature/makefile_fix

Feature/makefile fix
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-112
moved grad_log_prob method to static util log_prob_grad, made most model methods and get_base1 const, removed get_base1 assign tests, added get_base1_lhs tests