Fixed typo in comment
Column 'username' is not needed for getting the digest

* Use PDOStatement::fetchColumn()
Replaced hardcoded table names with class variables

* Added optional parameter $calendarSubscriptionsTableName to
  Sabre\CalDAV\Backend\PDO::__construct()
Merge pull request #378 from janpieper/fix-typo-in-comment

Fix typo in comment
Merge pull request #379 from janpieper/drop-not-needed-column-from-query

Column 'username' is not needed for getting the digest (PDO)
Merge pull request #380 from janpieper/use-table-name-variables

Replace hardcoded table names in SQL queries