TextFileManager : read and write ok

Thanks to this class, you can :
- read a .txt file (choose path)
- get frequency of each letters (nb and %)
- write a .txt file (choose path)

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch FileReader
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#	new file:   Projet_Crytp/Projet_Crytp.iml
#	new file:   Projet_Crytp/src/Library/Alphabet.java
#	new file:   Projet_Crytp/src/Library/TextFileManager.java
#	new file:   Projet_Crytp/src/Test_TextFiles/ceasar.txt
#	new file:   out/production/Projet_Crytp/Library/Alphabet.class
#	new file:   out/production/Projet_Crytp/Library/TextFileManager.class
#	new file:   out/production/Projet_Crytp/Test_TextFiles/ceasar.txt
#	new file:   out/production/Projet_Crytp/project_crypto/Controllers/CryptoController$TextTestListener.class
#	new file:   out/production/Projet_Crytp/project_crypto/Controllers/CryptoController.class
#	new file:   out/production/Projet_Crytp/project_crypto/Models/Ceasar.class
#	new file:   out/production/Projet_Crytp/project_crypto/Models/WordToNormalize.class
#	new file:   out/production/Projet_Crytp/project_crypto/Views/Global.class
#	new file:   out/production/Projet_Crytp/project_crypto/Views/MainView.class
#	new file:   out/production/Projet_Crytp/project_crypto/Views/TestView.class
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	.idea/.name
#	.idea/compiler.xml
#	.idea/copyright/
#	.idea/encodings.xml
#	.idea/misc.xml
#	.idea/modules.xml
#	.idea/scopes/
#	.idea/vcs.xml
#	.idea/workspace.xml