Implement MtProto 2.0 (closes #484, thanks @delivrance!)

Huge shoutout to @delivrance's pyrogram, specially this commit:
pyrogram/pyrogram/commit/42f9a2d6994baaf9ecad590d1ff4d175a8c56454

telethon/extensions/binary_reader.py

@@ -56,8 +56,11 @@ class BinaryReader:
         return int.from_bytes(
             self.read(bits // 8), byteorder='little', signed=signed)
 
-    def read(self, length):
+    def read(self, length=None):
         """Read the given amount of bytes."""
+        if length is None:
+            return self.reader.read()
+
         result = self.reader.read(length)
         if len(result) != length:
             raise BufferError(