Use pandas' _constructor for metadata bookkeeping

[thvitt]

Since (I guess?) 0.16, pandas supports a _constructor property on DataFrame or Series subclasses that will be used when pandas needs to create a new version of one of its datatypes. This could be used to keep track of our metadata in an easier and more robust way than the current manual implementation, where, e.g. corpus / scalar will always create a plain DataFrame and we have to copy the metadata manually, by implementing something like:

class Corpus:

     _metadata = ['metadata']

     @property
     def _constructor(self):
         def constructor(*args, **kwargs):
	     result = Corpus(*args, **kwargs)
	     result.metadata.update_from(self)
	     return result
	 return constructor

     ...

It would require to get rid of much of the conversion and copying code, though.