Implemented setSecretAlphabet in KeywordCipher and ShiftCipher
Merge branch 'master' of https://github.com/tgm-mritter/Verschluesselung.git
KeywordCipher:
+ Rewritten core and added throwing of exceptions
+ Added a class description and some comments
- Removed test main

ShiftCipher: 
+ Added a class description
- Removed test main

+ Added test package for test classes