Bumped to Lucene 4.6 and ensured docValues support.
Now populates the crawl_dates field for hash duplicate records.

For hash-based duplicates, the indexer now does the right thing and
checks for existing crawl_dates before merging any more dates. Does not
handle revisit records as yet.

Solr config handling is also centralised.
Initial support for revisit records.
Switching title to single-value.
Default to turning on more features.