Add support for self message

I've add support for sending message to the object itself.
For example, when the input is like the following:

    object April Todd
    note left of April: Lunch is ready
    April->April: Todd, what are you doing?

And the diagram is here:

                   +-------+                       +-------+
                   | April |                       | Todd  |
                   +-------+                       +-------+
    -----------------\ |                               |
    | Lunch is ready |-|                               |
    ------------------ |                               |
                       |                               |
                       | Todd, what are you doing?     |
                       |-------------------------      |
                       |                        |      |
                       |<------------------------      |
                       |                               |