issue0011: Fix muti download progress struck

if a task with empty urls list, it will struck. so cancel and send Stop && Error signal when it start

Change-Id: I2a7184ccab93cb0cc98e16e3dff3f5730417ab12
issue0012: add ftp soupport

Change-Id: Ifefe0f25ee80fe3b64fecf550d301342b866414a
issue0013: fix path structure

Change-Id: I5aa31daec7f990931c7c7ec7cc0b4b57b5ea9d3a
Merge branch 'master' into develop