[#10] Improve harvested metadata API

Some improvements on the endpoints that return the contents of the
harvest objects:

* Nicer URLs with redirects to the old ones
* Returning the raw harvest object content is available on the main
 harvest extension, so just redirect there
* Support for showing the original document of a harvest object, if
 present
* Suport for defining a custom XSLT for the HTML view, via

ckanext.spatial.harvest.xslt_html_content
ckanext.spatial.harvest.xslt_html_content_original
Merge branch 'release-v2.0' into 8-new-spatial-harvesters

Conflicts:
	README.rst
[#8] Do not add XML declaration when storing the content

Rather than store the XML declaration in the DB, we add it if not present
when outputing the contents (Also in ckanext-harvest's show_object)
[#8] Fix typo in GeoJSON template