Refactoring for NetStreamClients

There are many similarities between all clients which
use a NetStream to playback its video. Gather all these in
a generic class 'NetStreamClient' and inherit from this.
Clients: Remove ClientEvent.ABORTED

No use for this - it's really a ClientEvent.STOPPED. Whatever
extra error-management is needed should be done in the client.
MJPEGClient: Support for x-mixed-replace JPEG over HTTP

Motion JPEG can now be played if it is transmitted using HTTP
with content-type 'x-mixed-replace'. It will show each image
after each other as soon as the server sends them. The
rate control is effectively controlled by the server.

Seeking and buffering wont work at all. There is no flow control.
As soon as an image is ready, it will be displayed.

Pause works by simply disregarding all new imaged. It will
not actually stop the network stream.
Merge branch 'httpm'

* httpm:
  MJPEGClient: Support for x-mixed-replace JPEG over HTTP
  Clients: Remove ClientEvent.ABORTED
  Refactoring for NetStreamClients