ENH table metadata parameter that is copied with table operations
It's often useful to have some sort of info associated with a table. It would be handy if pandas Panel/DataFrame/Series objects had an empty `metadata` field that could be set to arbitrary values by a user, and for this metadata to be preserved across operations - especially ones which return copies of pandas objects, since if you just dynamically add metadata to an object it will not be preserved if you do an operation that returns copies or slices of a dataframe. 
