First draft of the Harvesting extension

This extension will hold all the harvesting code that is now located in
different parts of the ckan core and ckanext.dgu. This revision still
uses the ckanext-dgu forms API.
Controllers and templates have been defined for listing, creating and
showing the details of harvesting sources.
Ignore swap files
Use urllib2 and better error handling
Add a link to the create page
Fix bug in details template
Add controller to create a harvesting source

Before sending a create request to the forms API, an authz header is
added, with the API key defined in the config file
(ckan.harvesting.api_key). Also the data is formatted as the JSON object
expected by the forms API.
Send the authz headers on all requests
Harvesting jobs creation controller
Add refresh link to the UI.
Update README
Show error notices for duplicate sources and existing jobs.
Fix typo
Add option to delete harvesting sources
Add option to edit harvesting sources
Hide delete links for now
#1030 Move paster harvester CLI command to ckanext-harvest
#1030 Move harvesting model to ckanext-harvest
[security] change public file path code to avoid a trailing , and thus adding / as a static file app in the Pylons middleware cascade
[merge]
#1030 Move harvesting controller to ckanext-harvest