Add build artifacts and files to gitignore (#154)

Author: SaschaWillems

.gitignore

@@ -11,4 +11,10 @@ ebook/*.epub
 
 convert.py
 
-attachments/build/
+attachments/build/**
+attachments/android/.gradle/**
+attachments/android/.idea
+attachments/android/.gradle/**
+attachments/android/app/.cxx/**
+attachments/android/app/build/**
+local.properties