don't remove the test folder with make clean-all; only its contents

This change allows an individual to make STAN_HOME/test a symlink
to a directory that sits on a faster disk or a tmpfs.
adding the test for command line args to test-unit
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/vari-delete-fix
add no-op virtual delete() method to chainable() so that ctor exceptions can be cleaned up without seg faulting
Merge pull request #72 from stan-dev/feature/add_command_test_to_unit_tests

adding the test for command line args to test-unit
Revert "Merge pull request #64 from stan-dev/feature/mcmc_writer"

This reverts commit a0132a14c120266c6c2bc25c72242f44bb9951c2, reversing
changes made to e83baa2577f8abadede1f65a9ac2b0474de568ac.
Merge pull request #78 from stan-dev/feature/revert_pull_request_64

Revert "Merge pull request #64 from stan-dev/feature/mcmc_writer"
Revert "Merge pull request #72 from stan-dev/feature/add_command_test_to_unit_tests"

This reverts commit 2d0cafe9071b1704787717ac7f71ca5ef7a6583f, reversing
changes made to a0132a14c120266c6c2bc25c72242f44bb9951c2.
Merge pull request #79 from stan-dev/feature/revert_pull_request_72

Revert "Merge pull request #72 from stan-dev/feature/add_command_test_to...
Merge branch 'develop' into gp_improvements
Merge pull request #76 from stan-dev/feature/vari-delete-fix

Add delete method to chainable to allow exceptions in vari ctors
Merge branch 'develop' into gp_improvements
Merge pull request #58 from mbrubake/gp_improvements

Improvements to Gaussian Process models
Merge pull request #68 from stan-dev/feature/tweak_make_clean-all

don't remove the test folder with make clean-all; only its contents
Merge branch 'develop' into bfgs_improvements