[#24] Add command for pycsw integration

Based on the work by @kindly on https://github.com/kindly/ckan_pycsw_import,
adds a command to the extension that allows to export easily the spatial
datasets to pycsw. This will only work for harvested ISO datasets for the time
being.

The command can be run both via paster (in the context of a CKAN install) or
independently (eg a separate server), as it uses the API to get the datasets.

TODO docs
[#24] Add clear command to empty the pycsw table
[#24] Capture error when we get a wrong API response
[#24] Update API query to make it compatible with older Solr versions

The following expression is not valid in Solr 1.X:

"q":"harvest_object_id:*"

It returns the following error:

'*' or '?' not allowed as first character in WildcardQuery

It has been replaced with:

"q":"harvest_object_id:[\"\" TO *]"
[#24] Escape properly expression in last commit
[#24] Document CKAN-pycsw integration

Ad overview of how it works, how to set it up and deploy it in a
production server.
[#29] Use new precedence feature for previews.
[#29] Fix missing variable
[noticket] Notes about CSW limitation. Example GetRecords.
[#31] Check if sort parameter exists
[#28] Fix function name
[#32] Do not set height to avoid scrollbars if container is less than 600px
[#26] Fix broken markers on debug mode
[#27] Remove spatial_query_widget mention from README
Merge branch '29-new-preview-interface'
[#32] Add note about resource_proxy
Modify OWSLib requirement to prevent conflicts

We will target a specific changeset of OWSLib for the time being, just until
there is a proper release which includes geopython/OWSLib@4b0a62cd.

Basically OWSLib required python-dateutil==2.1, which is intended for
Python 3 and conflicted with the version used by CKAN core.
[#35] Ignore auth when using site_user
[#34] Update lxml requirement

Latest OWSLib introduces changes that require lxml>=2.3.
Merge branch 'master' into 24-pycsw-integration-command

Conflicts:
	README.rst