History of file versions at same location.

[JasperVanDenBosch]

fix #61

• [x] add()
• [x] series.addFile returns series
• [x] getSeries should return None inmediately if no seriesUid
• [x] get rid of "knowsXYZ" methods and tests
• [x] test_byLocation_works_for_file_in_series
• [x] byLocation, getSeries, etc should not inform listener or throw error??
• [x] web views should be mindful of repo retrurning None
• [x] listener.fileAdded
• [x] discovery adapt to status
• [x] status = new
• [x] status = series
• [x] status = new version
• [x] BaseFile.keepVersionsFromPrevious()
• [x] history access on instance
• [x] adapt approval
• [x] JsonFile serialize versions
• [x] mergeWith
• [x] get rid of timedeltas
• [x] history browsing on web view; show list of added datetimes;

[JasperVanDenBosch]