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
ParsedFEMFunction bugfixes
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.
Define NaN for ParsedFEMFunction
Merge pull request #252 from libMesh/dirichlet_fem_boundary

Dirichlet FEMFunctionBase Boundaries