Added Base#to_json and Base#attributes.

`ActiveFedora::Base#attributes` is required to support the #to_json
method, but it's useful in it's own right.

In order to accomplish this, we had to remove auto-generating datastream
accessor methods when `ActiveFedora::Base#add_datastream` is called.  If
you want an accessor for your datastream (and you aren't using `has_metadata`
or `has_file_datastream`) then you should call
`self.class.build_datastream_accessor(dsid)`
Merge pull request #296 from projecthydra/to_json

Added Base#to_json and Base#attributes.