* Added installation date and time to the database
Information on the date and time of installation of a spec is recorded
into the database. The information is retained on reindexing.
* Expose the possibility to query for installation date
The DB can now be queried for specs that have been installed in a given
time window. This query possibility is exposed to command line via two
new options of the `find` command.
* Extended docstring for Database._add
* Use timestamps since the epoch instead of formatted date in the DB
* Allow 'pretty date' formats from command line
* Substituted kwargs with explicit arguments
* Simplified regex for pretty date strings. Added unit tests.
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