[#63] Preview ArcGIS online maps

Use the embed option on the remote site, as embedding the page directly
did not work.
The viewer forArgis map services can not be embed because maps need to
be saved on the remote site.
Update the descriptions of arcgis based formats (needs reindex).
[#63] Advanced map viewer page

Adds a dedicated page for the advanced map viewer, which embeds the GMU
viewer but laoding it on the advanced mode. A link to it has been added
to the resource page for those formats that can be previewed.
Adds disclaimer on top of /datasets page
Added IE<9 hack to make location dropdown work
Removes beta, icon and changes URL and label for instance logo
Removes powered by CKAN from footer
Revert /dataset disclaimer string
Update ckanext-spatial branch requirement
Show thumbnail on dataset page if present
Add a metadata_type=geospatial extra to harvested geo datasets
Category tag additions
Merge branch 'master' of github.com:okfn/ckanext-geodatagov
add data.json import and api
extras rollup
[#66] Adds a load of accessibility fixes
[#68] Adds a load of UI tweaks
Add metadata type facet
Merge branch 'master' of github.com:okfn/ckanext-geodatagov
Merge branch '63-map-viewer-enhancements'

Conflicts:
	ckanext/geodatagov/plugins.py
Merge branch '66-accessibility-fixes'