Adding ParsedFEMFunction subclass

This needs some work to handle vector-valued elements, but for now it
ought to be a decent way to give parsed functions in configuration
files access to your solution variables.
Allow FEMFunctionBase data in DirichletBoundary
Bugfixes for FEMFunctionBase dirichlet BCs
ParsedFEMFunction bugfixes
Update traffic.
Use libmesh_error_msg() where appropriate.
Use libmesh_error_msg() where appropriate.
Use libmesh_error_msg() where appropriate.
Update traffic.
Update weekly traffic plot.
Add new fparser auto-differentiation header to autoconf install scripts.
Ran include/libmesh/rebuild_makefile.sh
Ran bootstrap.
Updating references

* Gimenez, 2014
* Schillinger, 2014
ParsedFEMFunction bugfixes
Merge pull request #256 from libMesh/fparser_intree

Fparser intree
Courty, 2014 preprint.
More FEMFunctionBase bugfixes
Fail gracefully on first System::init()

We can't construct a FEMFunction-based Dirichlet boundary before we
have an initialized solution to examine, but we shouldn't actually
*crash* in that situation.
Update traffic.