Added SHA512 for entry
Merge pull request #16 from matthiasbeyer/add-entry_hash

Added SHA512 for entry
Removed all old stuff from README
Added limit superclass method for querying the command if it allows searching in a certain dir
Added LimitRangeCommand class implementation
If we have no month or day, we use the first one, to search the complete year/month
Parse the start date and end date as Date objects, too
Implemented #search_in? method for checking if searching in a certain sub dir
Outsourced parsing of start and end dates in own method
Implemented LimitInCommand, which should inherit from LimitRangeCommand
Implemented LimitYearCommand class
Implemented LimitMonthCommand
Implemented LimitDayCommand