[#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.
Merge branch 'master' into 24-pycsw-integration-command

Conflicts:
	README.rst
[#24] Minor tweaks in README
[#24] Let pycsw define and create the table
[#24] Mention required pycsw version in readme
[#24] Readme: install Postgis and pycsw field length errors
Merge branch '24-pycsw-integration-command'
[#36] Fix spatial query widget for CKAN 2.1
[#37] Common function for initializing maps

This avoids duplication, mkaes all map consistent and makes easier
future support for other base maps.