Disambiguate calculate_norm(vec, H1)

Using system_norm.h instead of a forward declaration in system.h
ensures that the implicit conversion between enum FEMNormType and
class SystemNorm will be considered.

Removing the optional arguments from the single-variable
calculate_norm() ensures that the FEMNormType *won't* be implicitly
converted to an integer and interpreted as a variable index.
Fix dependencies

SystemNorm doesn't depend on system.h, but some of the error estimator
headers were depending on things indirectly via system_norm.h
Merge pull request #220 from libMesh/calculate_norm_disambiguate

Disambiguate calculate_norm(vec, H1)