Log Message:
Update to 1.0.15. From the changelog:

flint backend:
* Backport the lazy update changes from 1.1.2:
  WritableDatabase::replace_document() now updates the database lazily in
  simple cases - for example, if you just change a document's values and
  replace it with the same docid, then the terms and document data aren't
  needlessly rewritten.  Caveats: currently we only check if you've looked at
  the values/terms/data, not if they've actually been modified, and only keep
  track of the last document read.
* Fix PostingIterator::skip_to() on an unflushed WritableDatabase to skip
  documents which were added and deleted since the last flush.  (ticket#392)

documentation:
* Overhaul the doxygen options we use and tweak various documentation comments
  to improve the generated API documentation.
* Explicitly document that an empty prefix argument to
  QueryParser::add_prefix() means "no prefix".
* Update the documentation comments for Enable::set_sort_by_value(),
  set_sort_by_value_then_relevance(), and set_sort_by_relevance_then_value() to
  mention sortable_serialise() as a good way to store numeric values for
  sorting.