add close sas session function

[clarkliming]

saspy.SASsession has a method endsas to close the SAS session. by default it is not executed in del method, and this can lead to remote SAS session not closed. in saspy, I think the author want the users to use context like

with saspy.SASsession() as session:
    session.do_something()

since we are storing the SASsessions somewhere, I think we should call the endsas method when R exits. In addition, it is also possible for users to manage these sessions too

[tomweber-sas]

FYI, endsas() is driven be del(SASsession). It's in all access methods. del is still dependent upon garbage collection actually happening, so just submitting del(sas) doesn't force my code to go through there till gc is driven. But when the Python process ends, I make it go through del before releasing, so that all connection are terminated before Python itself terminates. Hope that answers this for you.
Tom