Fixed issues and cleaned up code.

Fixed typos.
Fixed doctring format.
Added get_flow_type method to subclasses, and removed it
from the base class.
Changed B_TO_Mb to B_TO_MBITS.
Combined send_next_data() and resend_data() to single method
called send_data(). Also, old packets are not stored any more.
Removed old_packets attribute, added batch_timestamp attribute.