Fixes detached persistency bug
manuell ausgeliehene TeachingMaterials werden als BorrowedMaterials
persistiert.
Collection mit allen TeachingMaterial bereitgestellt.
Merged #264 and upstream/develop.
Merge pull request #276 from Junkern/issue#209

Fixes detached persistency bug
Merge pull request #278 from davvitt/issue#271-manuelleLeihe

issue#271-manuelleLeihe
Merge pull request #279 from B3nRa/issue#264

Merged #264 and upstream/develop.
Added printing of PDF