Show only a sample or a prep metadata summary

The metadata summary handler is now capable of rendering only a sample template
or a prep template. Additionally I've fixed a bug with the wording of the
summary results.

@antgonza, I am not plugging this with the GUI, but you should be able to do go
to the following URL if you want to look at a sample template:

http://localhost:8888/metadata_summary/?study_id=10000&sample_template=10000

And to this URL if you want to look at the prep template:

http://localhost:8888/metadata_summary/?study_id=10000&prep_template=1
I prefer the classic:
`prep_template = self.get_argument('prep_template')`
at the beginning of the code so we know from the beginning what to expect for that function.