Update the test_headers make target slightly

This patch adds:

1.) The ability to test a specific subset of headers by running:
    make HEADERS_TO_TEST="numerics/parsed_function.h base/dof_map.h" test_headers

2.) Header tests in both opt and dbg mode, as opposed to devel mode.
    Since devel mode sets neither -DDEBUG nor -DNDEBUG, it misses both
    "#ifdef DEBUG" and "#ifdef NDEBUG" sections of the code, so
    testing both opt and dbg modes is more comprehensive.
rework to avoid GNU-make extension
Merge pull request #268 from libMesh/test_headers

Update the test_headers make target slightly
allow installcheck include and examples targets to do some parallel forking
tone down AutoDiff error reporting and make it switch-offable
Merge pull request #271 from dschwen/erroronce

Tone down AutoDiff error reporting and make it switch-offable
Use the environment's value of JOBS, if set
Merge pull request #270 from libMesh/parallel_install_tests

allow installcheck targets to do some parallel forking
detect cJump opcodes (use of "if")
Update traffic.
Merge pull request #273 from dschwen/cjump_error

Detect cJump opcodes in FParser AutoDiff
Construct NemesisIO single precision vectors in one line.
Fix incorrect assert.
Merge branch 'nemesis'
Update references

* Prudencio, 2014.
* Vallaghe, 2014.
* Vasconcelos, 2014.
Add Cournoyer, 2014.
Update citations.
Fix compilation failure in fparser

When that debugging cpp symbol was unset, we were trying to
instantiate template functions that hadn't been defined.
Project adjoint solutions with proper constraints

Previously if anyone tried to use an adjoint solution after a system
reinit() they'd have found it to have been corrupted by any *primal*
Dirichlet BCs.  Since correctly projected adjoint solutions might be
useful (if only as an initial guess for the fine grid adjoint solve),
let's make sure they get the correct adjoint BCs.
Project adjoints correctly in reinit()