Support wildcard CommonDatasets (netcdf4-python has MFDataset)

Includes a simple test
.getncattr shouldn't really be used here.  Just use getattr.
MFDataset sometimes will require a str input, not unicode
Catch a IndexError when trying to load a MFDatasets
Add back in the opennc function.  It is used in paegan-transport.
Fix exception logging.  I'm not sure what I was thinking.
Don't reopen a netcdf4-python dataset if it is already open.

Some large datasets can take many seconds to load and we were reopening
them every call.  This was designed as a function to be called fairly
often to ensure the datasets was open.
Merge pull request #70 from kwilcox/master

Local file(s) support and cleanup