Move all enumerations into the libMesh namespace

We've been doing something kind of bad for a while now: all of our enumeration header files had
```
using namespace libMeshEnums;
```
in them, violating the general rule of not putting `using` declarations in headers.  We actually ran into a namespace collision while trying to create a class called "VTK", which collided with one of the IOPackage enumerations.

This patch moves the enumerations into the libMesh namespace and cleanses the rest of the library of the libMeshEnums namespace.  It's possible that this change will affect applications, but the fix is trivial: just remove `libMeshEnums::` wherever it appears in your code.

This was actually Roy's idea so I've discussed it with him already, but I wanted to give others the chance to comment... for example, does anyone know why we had this separate namespace in the first place?