Add SLA to skinny.

Adds sublines to skinny lines. Each subline can be attached to an 
SLA station/trunk combo. Includes the following functionality:

Callid is persistent for both in/out calls on all skinny devices.
Can join, hold, resume.
All sublines appear under a single line button.

See: https://wiki.asterisk.org/wiki/display/~wedhorn/Skinny+SLA for doc.

(closes issue ASTERISK-17947)

Review: https://reviewboard.asterisk.org/r/1239/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@328381 65c4cc65-6c06-0410-ace0-fbb531ad65f3