Create Database abstract class and make a sqlite example for it

src/educoder/database/database.py

@@ -0,0 +1,3 @@
+class Database(object):
+    def connection(self):
+        pass

src/educoder/database/database_sqlite.py

@@ -0,0 +1,10 @@
+import database
+import sqlite3
+
+
+class DatabaseSQLite(database.Database):
+    conn = sqlite3.connect
+
+    def connection(self):
+        self.conn = sqlite3.connect("../../../data/lib_manager.db")
+