Make 'SolrConnector.search' honor 'fl' argument to limit returned fields.
Refactor purge_removed_docs_from_index():

Purge docs for a single site instead of a list of sites,
and have indexed docs passed in instead of fetching them
from Solr from within the function.
Switch to Solr single-core buildout for development.
Add mailchimp.com to the testing config because

it doesn't provide 'lastmod' in sitemap but sends
HTTP Last-Modified header and supports conditional
If-Modified-Since GET requests.
Use connection pooling for 'ResourceFetcher' requests.
Add helper function to parse and create RFC 2616 compliant HTTP dates.
Skip URLs that attempt to redirect.
Only fetch resources if modified since last indexed.
Add command line argument to only index a specific URL.