Release v0.0.4: add a new field to store the last contact date

This field is used to order in ascending order records by
the last contact date.

The last contact date is extracted from several possible fields,
and if none of them is available, from the 'modified' field.

New fields added by the indexer are prepended by convention with 'index'
Merge pull request #9 from emig/773/search_implementation

closes #6 Release v0.0.4: add a new field to store the last contact date